Jeremy Corbyn said he had changed his mind on a snap election after the EU gave the UK another Brexit extension
The Labour leader said this took the risk of a no-deal Brexit off the table - his main objection to the PM's plan.
The prime minister can only hold an election with the support of MPs - who have blocked it three times.
He is currently making a fresh attempt to get their backing in Parliament.
MPs are debating a government bill for an early election on 12 December, which requires only a simple majority of one to pass.
Speaking in the Commons, Mr Johnson said it was time to "refresh this Parliament and give the people a choice" about the future of Brexit and the country.
Mr Corbyn said Labour would back any moves by MPs to give 16 and 17-year olds and EU nationals with settled status the right to vote - a move strongly opposed by the government.
"We want the election to involve as many people as possible," he said.
The Scottish National Party and Liberal Democrats want a 9 December poll, saying it would prevent the prime minister from pushing his Brexit deal through Parliament.
No 10 sources have told the BBC they would accept 11 December to get opposition parties on-board - and they have agreed to put Brexit legislation on hold, for now.
Labour has, against the wishes of many of its MPs, shifted to supporting a December election and with that, it means we are on for the first December general election in decades.
The prime minister hopes this will give him a victory at the polls that would allow him rapidly to get his Brexit deal through Parliament and the UK out of the EU.
The Labour leader hopes for a souped-up version of his move forward at the 2017 election that would mean, contrary to the view of many of his own MPs, his project can continue and build.
The Lib Dems and SNP hope for a chance to stop Brexit happening, and expand their own political positions at a junction for the country.
But none of the parties can be remotely sure of what will happen next.
Read Laura's full blog.
Mr Corbyn was cheered by members of his top team, as he made his announcement at Labour's campaign headquarters in central London.
He said: "I have consistently said that we are ready for an election and our support is subject to a no-deal Brexit being off the table.
"We have now heard from the EU that the extension of Article 50 to 31 January has been confirmed, so for the next three months, our condition of taking no-deal off the table has now been met.
"We will now launch the most ambitious and radical campaign for real change our country has ever seen."
Mr Burgon said Labour would be pushing to get votes for 16 and 17-year-olds, as well as EU nationals so they can have their say.
But he added: "Even if we don't get those things we want, when push comes to shove, we are going to support an election."
However, not all Labour MPs are on board, with Ben Bradshaw saying it was a "bad mistake" and calling instead for another referendum on Brexit.
His fellow backbench MP, Barry Sheerman, tweeted that it was "sheer madness" to hold a December election "on Boris Johnson's agenda".
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