Jeremy Corbyn has tabled plans for a two-year Brexit delay to avoid the UK leaving the EU without a deal.
The Labour leader's formal blueprint - which could extend a key deadline to 1 January 2023 - is one of his final interventions in Parliament before he steps aside for a new leader in the spring.
But it is doomed to fail because the Conservatives have an 80-seat majority.
Backed by leadership candidate and Shadow Brexit Secretary Sir Keir Starmer, Mr Corbyn has laid down an amendment to the Withdrawal Agreement Bill - which is due to pass through Parliament this month.
The amendment would overturn Boris Johnson's vow to stop any extension to the 'transition period'.
The UK will leave the EU on January 31. But during the transition period, which lasts until 31 December 2020, the UK will continue to follow EU rules while London and Brussels pull together a deal on future trade.
The EU had said the transition period could be extended to give both sides more time.
But Boris Johnson has vowed to block any extension and even made a delay illegal - despite most experts warning he will not be able to get a full-blown trade deal by December.
Labour say that will leave Britain on the dangerous cliff-edge of a no-deal Brexit by Christmas this year.
So Labour's amendment would force Mr Johnson to ask for a two-year extension to the transition period if he has not "concluded" an agreement on future trade with the EU by June 15.
However, there is also a get-out clause in Labour's amendment.
A delay could be avoided if the Commons passes a motion backing an exit on December 31, 2020.
This means that even if Labour's amendment passes, which seems impossible as it is due to the Parliamentary maths, it would not necessarily delay Brexit anyway.
Labour say the amendment is simply designed to "restore the role for Parliament" in deciding if no-deal should be a factor.
And they say it would eliminate the "trapdoor" of no-deal Brexit on 1 January 2021 without MPs having any say.
