Shadow Brexit secretary Sir Keir Starmer told MPs the Government’s handling of negotiations had placed the UK in a position where the only option may be to extend Article 50 and move the 29 March exit day back.
The Labour frontbencher also shared his belief it was not “viable” for the UK to leave with no deal.
Sir Keir, speaking in the Commons said: “There is a question of extension of Article 50 and that may well be inevitable now given the position that we are in, but of course we can only seek it because the other 27 have to agree.”
But the Labour frontbencher questioned whether the EU would have any “appetite” to renegotiate even if Article 50 was extended.
He said: “I have to say and, with regret, I genuinely think that the way the Government has gone around the negotiations has undermined a lot of the goodwill that otherwise would have been there.”
Sir Keir, on a no deal, added: “I actually genuinely think we can’t do it on 29 March this year. It’s simply not viable, for so many practical reasons.”
The comment prompted the Father of the House Ken Clarke to ask whether Labour would work with Tory Remainers to ensure a “form of customs union and sufficient regulatory alignment to keep open borders” with the EU.
“We’re going to have to have a discussion, I think next week starting after Tuesday about where we go next and we’re all going to have to enter that in the right spirit,” Sir Keir said.
Tory Sir Oliver Letwin (West Dorset) echoed Mr Clarke’s request, asking: “Now I’m one of those in this House who would like some way or another at some point or other in the not too distant days to arrive at some cross party agreement about something we could actually go forward with and therefore I ask him whether he could explain to the House what is a strong single market deal that would need to be delivered in order to get an agreement?”
Sir Keir replied: “Is it possible, would the EU be interested in a discussion about what that sort of working customs union would look like? I’ve actually had the discussion.”
Independent Ulster Unionist MP Lady Sylvia Hermon (North Down) later demanded to know why Sir Keir wanted to vote down the Withdrawal Agreement when it contained provision to protect the Good Friday Agreement (GFA).
She said: “That [agreement] put an end to the appalling violence of 30-plus years in Northern Ireland where 302 police officers lost their lives, thousands of innocent people lost their lives in a terrorist campaign.
“Could he explain to this House and to the Irish diaspora in Labour constituencies how it is that the Labour Party is voting down this deal which guarantees the [Good Friday] Agreement.”
Sir Keir responded: “I don’t think it’s fair to characterise anybody who says these two documents are not the right deal for our country on the basis that that is to undermine the Good Friday Agreement because that simply means there can be no criticism, there can be no issue, there can be no challenge to the Government and that cannot be right.”
Speaking later outside the chamber, Labour MP Ian Murray, a supporter of the Best for Britain campaign for a second referendum, said: “This is a hugely significant moment, marking the first time a shadow cabinet member has acknowledged that the shambolic Brexit process means the 29 March deadline is unrealistic.
“This is entirely the fault of Theresa May, who has failed to strike a deal that is acceptable to either Remainers or Leavers, and has delayed the vote in Parliament until the last possible minute.
“With the Prime Minister’s deal destined to be defeated next week, there is now an opportunity to stop Brexit for good through a people’s vote.”