Tom Watson, the deputy Labour leader, pictured alongside Jeremy Corbyn credit: Eddie Mulholland for The Telegraph

The plans suggest that under Labour the UK could remain in both the Single Market and the Customs Union indefinitely if the European Union agrees to reform free movement.

Now Mr Watson, the party’s deputy leader, has appeared to confirm that possibility.

Asked on BBC Newsnight if Labour was now the party of “soft Brexit”, Mr Watson replied: "Yes, you have seen Keir Starmer's statement, we think that being part of the Customs Union and the Single Market is important in those transitional times because that is the way you protect jobs and the economy, and it might be a permanent outcome of the negotiations, but we have got to see how those negotiations go."

The publication of Labour’s new plans prompted concerns from Brexiteers that the party was seeking to “wreck" Brexit.

Sir Keir Starmer, the shadow Brexit secretary credit: Jack Taylor/Getty Images Europe

Many of Labour’s traditional strongholds in the North overwhelmingly voted Leave and the adoption of the new Brexit policy has raised fears that such voters could abandon the party.

Announcing Labour’s change of tack on Brexit, Sir Keir said the party wanted to secure a transitional deal with Brussels “that maintains the same basic terms that we currently enjoy”.

He said: “That means we would seek to remain in a customs union with the EU and within the single market during this period. It means we would abide by the common rules of both."

And he added: "We will always put jobs and the economy first. That means remaining in a form of customs union with the EU is a possible end destination for Labour, but that must be subject to negotiations.

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"It also means that Labour is flexible as to whether the benefits of the single market are best retained by negotiating a new single market relationship or by working up from a bespoke trade deal."

Meanwhile, Yanis Varoufakis, the former Greek finance minister and left-wing ally of Jeremy Corbyn, said Labour’s Brexit approach is shifting towards advocating an even lengthier transitional period.

Outlining the different approaches to Brexit considered by the Democracy in Europe Movement 2025 group he co-founded, he told the New Statesman magazine: “The final option… was to support a ‘transitional period’ of five to seven years, and it is this that Labour is edging towards adopting.”

Mr Varoufakis also said he warned Mr Corbyn and John McDonnell, the shadow chancellor, that the two-year withdrawal period dictated by Article 50 “would prove insufficient”.

Mr Watson also insisted there was no plot to oust him from his deputy leader role after Scottish Labour leader Kezia Dugdale quit her post amid speculation her departure was prompted by clashes with Mr Corbyn.

Pressed on whether people were trying to get rid of him, Mr Watson said: "Well, there is always someone trying to get rid of you in politics, but no, I don't actually see any move to remove me from my position.”