BORIS JOHNSON has vowed to press on with leaving the European Union, saying that he is “undaunted” by Parliament forcing him to seek a Brexit extension with EU leaders.

In the first Saturday parliamentary sitting since 1982 Mr Johnson suffered fresh defeat after MPs voted to back the Letwin Amendment by 322 votes to 306.

It took place as a march some estimated as large as a million people demanded a second referendum on the EU, addressed by shadow cabinet members including John McDonnell and by former prime ministers John Major and Tony Blair.

The motion, which was tabled by Tory grandee Sir Oliver Letwin, who lost the Tory whip for rebelling against Mr Johnson last month, sought to guarantee that the PM would comply with the terms of Hilary Benn’s “Benn Act,” which rules out any possibility of a no-deal Brexit on October 31.

Under the Benn Act Mr Johnson had until 11pm on Saturday to write a letter that requests a delay to Brexit.

He wrote and sent the letter but did not sign it. He accompanied it with another letter — which was signed — expressing his view that a further extension would be a mistake.

The passing of the amendment scuppered plans to vote on Mr Johnson’s deal with the EU on Saturday night. But the PM’s allies hope they will still have time to get it approved by Parliament this week, while the government was pushing for a meaningful vote as early as today.

Foreign secretary Dominic Raab told the BBC’s Andrew Marr Show: “Notwithstanding the parliamentary shenanigans, we appear to have now the numbers to get this through.”

Chancellor of the Duchy of Lancaster Michael Gove told Sky News that Mr Johnson is “determined” to get the deal through Parliament and accused backers of the Letwin Amendment of wanting to “explicitly try and frustrate” Brexit.

But Labour’s shadow Brexit secretary Keir Starmer said the party wants to legislate to avoid a “trapdoor” no-deal Brexit at the end of 2020 — something he says he fears is likely if no further deal is reached at the end of the transition period.

Mr Starmer also told Mr Marr that Labour would support an amendment that would require the deal to be put back to the public in another referendum, saying: “It’s got to go back to the public.”

Labour MPs Kevin Barron, Ronnie Campbell, Jim Fitzpatrick, Caroline Flint, Kate Hoey and John Mann voted against the amendment.

Labour’s Melanie Onn, whose Grimsby constituency voted Leave, abstained.

Eurosceptic Labour MPs Laura Smith and Dennis Skinner voted in favour of the amendment, with Mr Skinner saying: “I’m voting because the Tories are getting rid of workers’ rights. That’s enough for me.”

But Ms Smith tweeted criticism of the EU on Sunday, warning: “The EU is no beacon of workers’ rights,” as she slammed the European Court of Justice for its pro-business rulings.

The EU has said that it is waiting on Britain to “inform it of the next steps.”