With some polls giving Ms May a more than 20 point lead before the June 8 vote, the main opposition Labour Party pledged on Sunday to introduce four new public holidays to try to unite a country left deeply divided by the Brexit vote.
The Liberal Democrats, who were a distant fourth in the last election, reiterated their message that they were the only "decent opposition" to a government it says is pursuing a 'hard Brexit', while the eurosceptic UK Independence Party said it would ban full veils worn by some Muslim women.
The early election, which stunned British politicians, could redraw the political landscape after Brexit exposed deep faultlines in Britain, with Scotland and Northern Ireland voting to remain in the EU while England and Wales supported an exit.
Some commentators see a dominant Conservative Party crushing left-leaning Labour - an all but 'coronation' of Ms May, who was appointed prime minister shortly after Britain voted to leave the EU in June last year, giving her a free hand in the talks.
"The four nations that make up our great country have rarely been more divided due to the damaging and divisive policies of this Conservative government," Labour leader Jeremy Corbyn said, announcing his plan to add new national holidays for the four patron saints of the British Isles.
The Liberal Democrats, the only party to criticise Britain's departure from the EU, said they would offer a second referendum on any Brexit deal, while UKIP denied it had tacked to the right with its proposal to ban face veils.
Tim Farron, leader of the Liberal Democrats, said voters would get a chance "to reflect on whether they like the (Brexit) deal" and to change the country's direction.
Britain can expect to hear more promises in the weeks before the election, but with Ms May way out ahead in the polls, it is unclear what impact they will have.
The Conservatives were also seen making inroads in Scotland, with pollster Survation saying they had opened up a 10 percentage point lead over the Scottish Labour Party.
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