Delaying Brexit “may well be inevitable now” because of Tory government chaos and parliamentary deadlock, Labour has warned for the first time.

In a major shift, shadow Brexit secretary Keir Starmer said that MPs would move to extend talks with Brussels rather than crash out of the EU without a deal on March 29. 

His remarks followed those of Jeremy Corbyn’s spokesman, who told reporters that the two-year ‘Article 50’ process leading up to Brexit could be prolonged. “Clearly there is a possibility that Article 50 may have to be extended,” he said.

At the start of a five-day Commons debate on Theresa May’s plans, Starmer said that it was time to face up to the reality facing MPs.

“The two-year window has effectively been rundown. There is a question of extension of Article 50, and that may be inevitable now given the position we are in. But of course we can only seek it, because the other 27 have to agree.” 

Starmer added: “I genuinely think that leaving with no-deal would be catastrophic. I actually genuinely think we can’t do it on 29 March this year.

“It’s simply not viable, for so many practical reasons. We’re going to have to look at what are the available options that realistically are still on the table.”

With parliament unable to find a majority for the kind of Brexit it wants, MPs believe that giving themselves more time is one option that could find support on all sides if May’s plan is defeated next week and no other alternative is found.

Suspicions intensified this week that May herself could seek to delay Brexit for a few months at least, with one minister admitting that it may be the only option if MPs fail to back the PM’s deal.

“If that proves to be impossible then I think we need to…we have very little time left but we might have to extend Article 50,” digital minister Margot James said.

In Prime Minister’s Questions, May refused to give to Tory backbenchers a ‘categorical’ guarantee that she would not delay the date of exit from the EU, saying only “I want” to leave on March 29 with her deal.

Earlier, Corbyn’s spokesman said: “We are not pressing for an extension of Article 50. If that has to happen that’s the responsibility of the government.

“We are in a crisis, as a result of their repeated failures. If it’s only possible to deal with that crisis through an extension of Article 50, it’s the responsibility of the government. Let’s see what happens next week.”

The Labour leader’s spokesman also stressed that the party’s preferred option was to hold a general election, but he admitted that it could even campaign to stay in the EU if enough members wanted that as policy.

“The manifesto that any Labour Party leadership stands on is always decided by internal party democracy. Procedures are set out in Clause V of our constitution.”

Asked if the party could campaign at the next election on a policy of remaining in the EU if enough members wanted it, the spokesman said: “The policy platform is set by the constitution.”

Under Labour’s policy approved at its conference last year, the party will try to trigger a general election – but if it cannot get one it will then look at other ‘options’, including a second referendum.

Some of Corbyn’s allies believe that those options include one last attempt to get the Commons to support Labour’s plan for ‘a customs union’. 

Tory grandee Ken Clarke opened up that prospect, asking 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.

Corbyn’s spokesman confirmed it was not automatic that a second referendum call would follow a rejected election move: “Other options mean putting our alternative plan to Parliament.”

Labour MP Ian Murray, of the pro-EU Best for Britain campaign, said: “This is a hugely significant moment, marking the first time a shadow cabinet member has acknowledged that the shambolic Brexit process means the March 29 deadline is unrealistic.” 

Two shadow ministers, Barry Gardiner and Andrew Gwynne, claimed on Wednesday that Labour would ‘immediately’ seek a confidence vote in the government if May was defeated next Tuesday.

In a speech in Yorkshire on Thursday, Corbyn will step up his calls for a general election, claiming it would heal divisions and “would give the winning party a renewed mandate to negotiate a better deal for Britain”.

His remarks appeared to underline his belief that Brexit should still happen but that Labour’s version would be better able to protect jobs and the economy.

Tory chairman Brandon Lewis said: “Labour simply do not have a plan for Brexit. Instead they are arguing in public about whether to frustrate the decision of the British people and rerun the referendum.”

And Green MP Caroline Lucas said the priority should be to get a second referendum. “I share Corbyn’s desire to get rid of this toxic Government – but we won’t heal the very real divisions Brexit has caused until we confront them.”