MPs are debating Boris Johnson’s call for a general election on 12 December after Labour threw its weight behind a pre-Christmas poll.
Jeremy Corbyn said he had changed his mind on a snap election after the EU gave the UK another Brexit extension.
The prime minister can only hold an election with the support of MPs – who have blocked it three times.
No 10 says it will abandon its latest attempt if MPs support giving 16 and 17-year olds and EU nationals the vote.
Labour has said it will back any amendment calling for these groups to be allowed to take part in the proposed election.
Mr Corbyn told MPs: “We want the election to involve as many people as possible.”
Downing Street said extending voting rights in this way would delay any election by as much as six months.
A Downing Street spokesman said: “Nothing would sum up how broken this Parliament is if after them all standing up there and saying they want a general election – they vote for an amendment which is deliberately designed to actually prevent the delivery of a general election before 2020.”
If amendments calling for votes at 16 and for EU nationals are selected, MPs will vote on them after 19:00 GMT, before being asked to give their final approval to a 12 December poll.
The Scottish National Party said MPs “should not be afraid” of expanding the franchise although it is unclear whether it and the Lib Dems will back the move if it is likely to scupper the chances of an imminent election.
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.
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…We will now launch the most ambitious and radical campaign for real change our country has ever seen.”
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”.