Boris Johnson says he plans to stand as leader of the Conservative Party.
The former foreign secretary told the British Insurance Brokers' Association conference today: "I'm going to go for it, but there is no vacancy at present."
Rumours Mr Johnson has had his eyes on the leadership have been circulating for some time - and he finally confirmed his intentions as Mrs May comes under increasing pressure to name the date of her departure as Prime Minister.
Mrs May told her party in December she would step aside once she has delivered the first phase of Brexit.
But mutinous MPs frustrated by her failure to deliver Brexit are urging her to lay out a specific timetable for when she will quit.
The Prime Minister attended a meeting of the 1922 Committee - an influential group of Tory backbenchers - this morning, which discussed her departure.
If she fails to nail down a date, a number of rebellious Brexiteers plot to change the rules of the party in order to be able to hold a vote to unseat her.
The Government is planning to hold a vote on the Withdrawal Agreement Bill - the piece of legislation that would enact Mrs May's agreement with the EU - in the first week of June.
Brexit Secretary Stephen Barclay told to the Lords EU select committee yesterday the vote could mark the death of Theresa May's Brexit deal.
He said: "I think if the House of Commons does not approve the bill then the Barnier deal is dead in that form and I think the house will have to then address a much more fundamental question between whether it will pursue a no-deal option or whether it will revoke."
Boris Johnson watches the election results with his wife Marina (L), at the count in Watlington, Oxfordshire, after winning the Henley seat for the Conservatives in the 2001 General Election. The seat was Michael Heseltine's, who has stepped down at this election.
Spectator editor and Conservative MP Boris Johnson (left) chats to former Tory Chancellor Lord (Norman) Lamont at a party to mark the Spectator's 175th anniversary, at the Four Seasons Hotel, Park Lane, London. Among those also in attendance at the party were Home Secretary David Blunkett, broadcasters David Dimbleby and Andrew Neil, and TV chef Nigella Lawson.
Boris Johnson, Conservative MP and editor of The Spectator magazine, at a party to mark The Spectator's 175th anniversary, at the Four Seasons Hotel, Park Lane, London. Among those also in attendance at the party were Home Secretary David Blunkett, broadcasters David Dimbleby and Andrew Neil, and Ulster Unionist leader David Trimble. 16/10/2004 The Spectator editor Johnson, has been ordered by Tory leader Michael Howard to go to Liverpool to apologise for an article in his magazine accusing its people of 'wallowing' in their 'victim status'. Mr Johnson, who is also the Conservative shadow minister for the arts, said he would be travelling to the city next week in a 'spirit of complete humility' to apologise in person for the offence caused.
Shadow Arts Minister Boris Johnson campaigns for donations from delegates at the Conservative Party Annual Conference in Bournemouth. 15/10/2004: Tory MP Boris Johnson who edits The Spectator magazine where a leading article has accused Liverpudlians of wallowing in what they regard as their "victim status" over the murder of Ken Bigley and the Hillsborough football disaster in 1989.
Conservative Party MP Boris Johnson sits through a series of speeches during the Conservative Party Conference in Bournemouth.
Boris Johnson MP (centre) and editor of the Spectator at the Liverpool Institute for Performing Arts during a visit to the city to apologise for remarks made in his magazine about beheaded hostage Ken Bigley and the Hillsborough tragedy. But council leaders called on the people of Liverpool to ignore the "Boris Johnson show". The Tory MP for Henley created a storm of protest after the publication of a leader written in the Spectator, which suggested Liverpudlians were "hooked on grief".
Boris Johnson MP and editor of the Spectator magazine arrives at Claridges Hotel in London, where he is hosting the 'Spectator Parliamentarian of the Year' Awards. 13/11/2004 Boris Johnson who was Saturday November 13 2004, sacked from the Conservative frontbench amid fresh allegations about his private life, a spokesman for Tory leader Michael Howard said.
Boris Johnson arrives at his house in north London, after going for an early morning jog. The former Shadow Arts Minister and Deputy Chairman of the Conservative Party, was relieved of his position by party leader Michael Howard, after revelations about his private life were published in a tabloid newspaper.
Boris Johnson, MP for Henley and Conservative spokesman on Higher Education, opens a footpath at the Environments Agency in Wallingford, Oxon, Friday April 7 2006. He later started a race between a car, a cyclist, runners and walkers to promote green travel. Watch for PA story.
England's Boris Johnson and Germany's Maurizio Gaudino shake hands at the end of the game
England's Boris Johnson (L) tackles Germany's Maurizio Gaudino during the Legends match at the Madejski Stadium in Reading.
Shadow education spokesman Boris Johnson MP, attends the final day of the Conservative Party's Annual Conference in Bournemouth.
Boris Johnson MP addresses a group of sixth form students from schools across London after being handed a petition against plans to scrap the last remaining ancient history A-level outside the House of Commons, London.
Boris Johnson (left) joins the Save our Services campaign against health cuts at St, Mary's Hospital in Sidcup, Kent.
Candidates for Mayor of London, Labour's Ken Livingston (left), Lib Dem candidate Brian Paddick (centre) and Boris Johnson of the Conservative party, appear together on television debate.
Conservative candidate for Mayor of London Boris Johnson launches the final phase of his mayoral campaign at Bounces Road Community Hall, North London today.
Conservative Party leader David Cameron congratulates Boris Johnson on becoming London Mayor at Tory HQ in central London.
Newly elected London Mayor Boris Johnson wears a policeman's hat in Trafalgar Square, central London during the Sikh New Year Festival celebrations of Vaisakhi.
Mayor of London Boris Johnson waves the Olympic Flag during the handover ceremony as President of IOC Jacques Rogge (centre) and Mayor of Beijing Guo Jinlong look on at the National Stadium during the 2008 Beijing Olympic Games, China.
Britain's Queen Elizabeth II talks to London Mayor Boris Johnson during the Beijing Olympics Team GB reception at Buckingham Palace in London.
Retired Formula 1 World Champion Mika Hakkinen (right) and current Formula 1 World Champion Lewis Hamilton (left) join Mayor of London Boris Johnson in backing an anti-drink-drive campaign at Potters Fields Park in London.
London Mayor Boris Johnson (2nd Left) celebrates St Georges Day in Leadenhall Market in the City of London today where he saw a festival of English food and drink.
Leader of the Conservative Party David Cameron (2nd right) with Mayor of London Boris Johnson (2nd left) as they celebrate St George's day in Leadenhall Market in the City of London.
Boris Johnson and TV star Kelly Brook launch the Mayor of London's Skyride, Peter's Hill steps, London.
London Mayor Boris Johnson with a sculpture of a blue cockerel as he unveils the next two commissions for the fourth plinth in Trafalgar Square, which includes the cockeral (pictured) by Katharina Fritsch and a boy on a rocking horse named Powerless Structures by Michael Elmgreen and Ingar Dragset.
Boris Johnson pictured with members of the Genesis Mas band, who will be performing at the Notting Hill Carnival this weekend, at Potters Fields in central London.
Mayor of London Boris Johnson (left) with Prime Minister David Cameron during the lighting of the Paralympic Cauldron in Trafalgar Square, central London.
London Mayor Boris Johnson boards one of the first trains of the new London Overground extension connecting Clapham Junction with Canada Water. ASSOCIATION Photo. Picture date: Monday December 10 2012. The new service will be London's first new orbital Journey times from south and east London. See PA story RAIL Link. Photo credit should read: Stefan Rousseau/PA
Actor Brian Blessed (left) and Mayor of London Boris Johnson sell poppies in Liverpool Street Station in central London with they collected money from commuters including nine year old Arthur Crowley (centre) from Salisbury.
Mayor of London Boris Johnson (centre) swings from a bus as Northern Ireland Secretary Theresa Villiers (left), Northern First Minister and DUP leader Arlene Foster and Minister of Enterprise, Trade and Investment Jonathan Bell (second right) look on during a visit to Wrightbus Chassis plant in Antrim.
Foreign Secretary Boris Johnson (left) and his Russian counterpart Sergei Lavrov during a press conference following their meeting in Moscow.
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Mr Johnson's opposition to Theresa May's Brexit deal has won him support among Eurosceptics and he is popular with Conservative Party members.
The latest poll from grassroots website Conservative Home had Mr Johnson as the most popular leadership candidate among members.
However his path to power could be blocked by a struggle to win the backing of some of his fellow Tory MPs.
Some Conservatives have gone as far as to say they would quit the party if he took over.
Other Conservatives who have thrown their hat in the ring for the next leadership contest include Leader of the House of Commons Andrea Leadsom, International Development secretary Rory Stewart and former Work and Pensions Secretary Esther McVey.