Party leaders will return to the campaign trail amid opposition fears of a Conservative landslide after the Tories made sweeping gains in the local elections.
Jeremy Corbyn admitted Labour faces a task on a "historic scale" if it is to regain power in the General Election on June 8 after losing more than 300 councillors on a calamitous day for his party.
With the Conservatives gaining more than 500 councillors - as well as winning tightly-fought mayoral races in the West Midlands and Tees Valley - Theresa May insisted she was taking "nothing for granted".
But the results were widely seen as vindication both for her tough line on Brexit - threatening to walk away from talks with the remaining EU if she cannot get a good deal - as well as her decision to call a snap election.
The Conservatives effectively obliterated Ukip - which won just one seat and lost more than 100 - as voters who had deserted the Tories for Ukip returned in their droves.
Mr Corbyn, whose leadership had been widely criticised from within the Labour ranks, described the results as "mixed" and claimed the party was "closing the gap" on the Conservatives.
He acknowledged, however, that Labour had lost "too many"councillors and now faced a mammoth job if he was to gain the keys of No 10.
"We have five weeks to win the General Election so we can fundamentally transform Britain for the many not the few," he said in a statement.
"We know this is no small task, it is a challenge on an historic scale."
The divisions in the Labour ranks were underlined when former shadow home secretary Andy Burnham - the party's newly-elected metro mayor for Manchester - failed to show up for a visit to the city by Mr Corbyn to celebrate his triumph.
Liberal Democrat leader Tim Farron claimed only his party could prevent a Conservative "coronation", despite its failure to make a hoped-for breakthrough in its former South West stronghold.
Mr Farron sought to regain the electoral initiative with the announcement of "flagship" plans to put a penny on income tax to fund a £6 billion-a-year cash injection for the financially-stricken NHS and social care services.
He said voters recognised the need to "chip in a little more" to address the "chronic underfunding" of healthcare, pointing to a recent opinion poll which suggested 70% of voters would back a 1p rise in income tax if the money was guaranteed to go to the NHS.
Under the Lib Dem plan, 1p would be added to the basic, higher and additional rates of income tax and the rate of dividend tax from the next financial year with the proceeds being "ring-fenced" for the NHS, social care and public health.
"The Liberal Democrats will rescue the NHS and social care. We are prepared to be honest with people and say that we will all need to chip in a little more," Mr Farron said.
His announcement was seized on by the Conservatives, who said that all the main opposition parties were now committed to raising taxes.
Treasury minister Jane Ellison said: "A vote for anyone other than Theresa May means you will pay more tax.
"Jeremy Corbyn, the Lib Dems and SNP will hit 30 million people in the pocket with higher income taxes."