Labour will support plans for a general election following pressure from the Lib Dems and SNP.
Jeremy Corbyn announced at a shadow cabinet meeting that its conditions to back the move had been met.
He told his shadow cabinet that Labour would launch the “most ambitious and radical campaign for real change our country has ever seen” now a no-deal Brexit was “off the table”.
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 January 31 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.”
Shortly after Momentum tweeted: “Labour are officially backing an election. This is the opportunity of a lifetime to put an end to the shambolic mess the Tories have made and return hope to millions. Let’s do this.”
But Matthew Oxenford, UK analyst at The Economist Intelligence Unit, warned that Jeremy Corbyn is giving the image that his party is in disarray.
He said: “Once the election takes place the particulars of the parliamentary procedures that began the process are mostly likely to be forgotten, but if Labour starts the campaign looking like they were forced into it, it could be a potent example other parties will use to show they are in too much disarray to govern.”
Corbyn’s move does not mean a December election is certain, with Labour is expected to support amendments to the Bill.
The party is understood to back a change to the proposed date which, if such an amendment is selected by the speaker, could be backed by the Liberal Democrats and the SNP.
Lib Dem MP Chuka Umunna said his party is against holding a general election on Johnson’s preferred polling date of December 12, telling BBC Radio 4’s Today programme: “It cannot be the 12th.”
However, Umunna signalled the Lib Dems could be flexible over their own preferred election date of December 9.
He said: “We will see what else they come forward with. We have got to break the gridlock.”
A Number 10 source said: “If there’s an amendment to the 11th we could accept.”