Labour leader Jeremy Corbyn has welcomed the snap general election, saying he would put the interests of the majority first and fight against cuts.
- Video report by ITV News' Correspondent Damon Green
His party is still smarting from some high profile defeats two years ago.
If Labour are to have any chance of returning to power one seat they have to win is Morley and Outwood - lost by Labour's Ed Balls by just 422 votes in 2015.
Though Labour is still divided over its support for the party leader, those who have faith in Mr Corbyn are positive they can win this general election.
Shadow Education Secretary Angela Rayner MP says she is "absolutely confident" that Labour will win "if we get onto policies and what we're actually about, who we're actually looking after".
Labour's election campaign co-chair Andrew Gwynne said the party "absolutely are ready" for the snap poll, which they had been planning for ever since Mrs May took office.
A manifesto will be agreed by the National Executive Committee "in the next few weeks", he said.
The Liberal Democrats are hoping for a revival after a humiliating election night in 2015 when 49 of their MPs lost their seats and then leader Nick Clegg resigned.
- ITV News Correspondent Geraint Vincent gets the reaction from the Lib Dems:
The party hope a June election will see anti-Brexit sentiment help them recover from the 2015 bloodbath.
Tim Farron has unashamedly positioned his party as the pro-EU choice, warning against a "disastrous" hard Brexit and campaigning to keep the UK in the single market.
But while those messages may prove popular in Remain-supporting areas, the party's appeal in former strongholds which voted to leave the European Union may prove more limited.
The party hailed a membership surge which saw more than 2,000 people join in the hours following Theresa May's shock announcement of a June 8 election.
But with just nine MPs, Sarah Olney's success in the Richmond Park by-election added to the eight survivors of the 2015 disaster, any chance of the Lib Dems returning to the parliamentary force they were in 2005 and 2010 appears unlikely.
The backlash in 2015, following the years in coalition with the Tories, saw them lose ground to the Tories in seats in south-west England and the London suburbs.
But in Cornwall, Devon and Somerset, areas which had been Lib Dem heartlands, voters backed Brexit and may prove unlikely to support Mr Farron's pro-Brussels message.
Former Liberal Democrat leader, Lord Ashdown says people in former stronghold areas may be changing their minds about Brexit: "I think people did vote for Brexit unquestionably, some, not many yet but I think that [people] are beginning to wonder whether they made the right decision. What people did not vote for is the hard brutal Brexit that Mrs May has chosen."
Mr Farron indicated the Lib Dem ambition was to prevent the Tories having an outright majority giving his party leverage over the terms of Brexit.
"If you want a Britain that is open, tolerant and united, this is your chance," he said.
"Only the Liberal Democrats can prevent a Conservative majority."