Voters go to the polls in local elections across England that will put Theresa May to her biggest electoral test since she lost her government majority last year.
London is the major battleground after recent polls pointed towards a substantial swing to Labour.
Jeremy Corbyn's party will be hoping to win control of Tower Hamlets and Barnet and possibly even the flagship Conservative councils of Wandsworth and Westminster.
But upsets are also possible around the country, with Labour also hopeful of taking power in town halls from Kirklees and Calderdale in Yorkshire and Trafford in Greater Manchester to Plymouth and Swindon in the south.
Most councils are counting votes overnight but others will declare results during the day on Friday.
Polls close at 10pm and the first results could begin to arrive at around midnight and are expected to be in full flow by 2am.
Controversial plans forcing voters to prove their identity before casting their ballot will be trialled at some polling stations.
Bromley, Gosport, Swindon, Watford and Woking are all running pilot schemes.
Ministers say the scheme will help combat electoral fraud but critics argue it will suppress turnout.
The elections will give voters a chance to pass their verdict on the Prime Minister's handling of Brexit, and Liberal Democrats are hoping their promise of a second referendum will help them regain Remain-backing Kingston and Richmond in south London.
Elsewhere, Conservatives may be boosted by Ukip's decline in the polls, with targets like Basildon or Great Yarmouth potentially up for grabs if the Eurosceptic party performs poorly.
Across England, more than 4,000 seats are being contested in around 150 councils - including all 32 London boroughs, as well as every ward in Birmingham, Manchester, Leeds and Newcastle.
Mayoral elections are taking place in Hackney, Lewisham, Newham, Tower Hamlets, Watford and the Sheffield City region, but there are no polls in Scotland, Wales or Northern Ireland.
Recent national opinion polls have seen Conservatives and Labour running neck-and-neck on about 40% each.
But a survey last week for Queen Mary University gave Mr Corbyn's party an overwhelming 51%-29% lead over the Tories in London - enough to deliver the best result in the capital for any party in more than 40 years.
Even if these figures are repeated on Thursday, the prize of Westminster - Tory since the council was created in 1964 - may remain beyond Labour's reach.
But Wandsworth, which has been in Conservative hands since 1978 and was once regarded as Margaret Thatcher's favourite council, may be more vulnerable to a Labour surge.
An unknown factor in Wandsworth will be the impact of new anti-Brexit party Renew, which is wooing Europhile voters with nine candidates in a borough which voted 75% Remain in the EU referendum.
Another long-standing Tory citadel, Kensington and Chelsea, is a wild card in 2018, due to local anger over the council's handling of the Grenfell Tower fire tragedy.
New party Advance Together has a slate of 19 candidates fighting on a platform of "working for the interests of all residents, not for a political party".
Labour has been seeking to dampen down expectations of significant gains, pointing out that the party did well the last time the seats were contested in 2014, at a high point in Ed Miliband's leadership.
The party's recent difficulties over anti-Semitism allegations may have undermined its prospects in key target borough Barnet in north London, which has a large Jewish community.
Other areas where a swing to Labour could deliver overall control to Mr Corbyn's party include Amber Valley in Derbyshire, Carlisle, Dudley, Walsall, Worcester and North-East Lincolnshire.
Councils to watch for possible Conservative gains include Portsmouth, Basildon, Thurrock, Peterborough and Colchester, all of which could see the party win overall control on a relatively small swing.
The British National Party may end with no representation on any council in the UK, as its sole current councillor is not defending his seat in Pendle, Lancashire.