Labour has dangled the possibility of delaying Brexit if the exit deal negotiated with EU leaders is rejected by MPs.

Sir Keir Starmer, the shadow Brexit Secretary, said a Labour prime minister would then – unlike Theresa May – “go back to the negotiating table” to try to strike better withdrawal terms.

Asked if that would mean the UK staying in the EU past the March 2019 exit date, while the negotiations continue, Sir Keir replied: “We have to get the best deal we can.

“We are talking about the tail-end of 2018. We would have a vote and we would go back to the negotiating table. That is the position that we want to adopt.”

Sir Keir attacked Ms May’s refusal to return to Brussels if her exit terms are rejected by MPs as a stubborn refusal “for the sake of my country to go back and renegotiate”.

However, it will not be possible to delay Brexit beyond the two-year timetable set out in the Article 50 withdrawal process without the agreement of other EU leaders and the European Parliament.

They have sent out mixed messages about allowing Britain to rethink – with some hints dropped that an attempt to delay would result in a tougher negotiating stance in Brussels.

Sir Keir is attempting today to clarify Labour’s confused Brexit stance, by insisting it would make access to the single market – not ending freedom of movement – its key objective in the talks.

He will commit the party to tearing up the Conservatives’ White Paper and Great Repeal Bill and replacing it with legislation safeguarding workers' rights, consumer protections and environmental standards.

Labour will also promise that the rights of EU nationals living in Britain would be guaranteed on the first day of a Jeremy Corbyn-led government.

The party insists it accepts last year's referendum result and does not want to overturn it, but is offering a less-damaging alternative to Ms May’s hard Brexit.

It would give top priority to retaining the benefits of the single market and the customs union – both of which the Conservatives have vowed to walk away from.

Speaking on BBC Radio 4’s Today programme, Sir Keir said: “We accept that unchanged single market membership is not a viable option, but we would want to leave the options on the table.”

He attacked Ms May for closing down her options, adding: “That leaves her in a weak bargaining position. She may be clear, but she is rigid and she is reckless.”

However – crucially – Labour has not explained its stance on ending freedom of movement, to limit immigration, which the EU has insisted cannot happen with full single market access.

Last night, Lord Mandelson laid bare Labour’s problems when, asked on BBC’s Newsnight programme what the party’s Brexit strategy was, replied: “Search me”.

The Liberal Democrats said Sir Keir had “torpedoed” the launch of Labour’s Brexit blueprint, which was an attempt to mask its support for the Tories’ strategy.