Labour support for a fresh Brexit referendum will be the only “remaining option” if Theresa May refuses to adopt the party’s own plan for withdrawal, Keir Starmer has said.

The party’s Brexit spokesman challenged Jeremy Corbyn by saying Labour is already at “the phase” of its policy which promised to consider “campaigning for a public vote”.

The Labour leader has been accused of backtracking on that policy, by vowing to table repeat votes of no confidence to try to trigger a general election.

But Sir Keir told a conference there were only “two remaining options”, if that failed, the first being the government accepting a “close economic relationship with the EU”, as Labour proposed.

“Secondly, as our conference motion sets out, the option of a public vote,” he added.

Speaking to the Fabian Society, Sir Keir backed Mr Corbyn by agreeing Wednesday’s failed no-confidence vote was “not the end” of attempts to secure an election.

But he argued moves towards a possible Final Say referendum should now proceed alongside those efforts, with just 69 days until Brexit.

“It’s now time for an open and frank debate about how we break the deadlock,” Sir Keir added.

And, to loud cheers, he said: “Our conference motion states that: ‘If we cannot get a general election Labour must support all options remaining on the table, including campaigning for a public vote’.

“That is a very important commitment. It’s a commitment to you, our members and our movement. And it is one we will keep.”

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On Thursday, Mr Corbyn angered some Final Say supporters by saying Labour would only consider backing a fresh referendum if the UK was on the brink of the “disaster of no-deal”.

He confirmed he would table repeat votes-of-no-confidence and push for a Commons vote on its own Brexit plan – a customs union, “strong” alignment with the single market and protected workers’ and environmental rights.

Only then, if “the country is facing the potential disaster of no-deal”, would Labour consider support for a Final Say public vote, Mr Corbyn said.

In his speech, Sir Keir said of labour’s plan: “From my conversations over the last two years, I know the EU would have considered it if the UK government proposed it.”

But he added: “Be in no doubt this approach involves trade-offs and compromises. It is far from perfect.”

And, on a fresh referendum, he said: “I know there is significant support for this in our membership, in many trade unions, among a number of Labour MPs......this has to be an option for Labour.”

Sir Keir also repeated that, with a raft of bills still to get through parliament before Brexit, departure was highly unlikely to be on 29 March, as planned.

“It seems inevitable to me that the government will have to apply for an extension of Article 50,” he said.