BRITAIN should “stay aligned” to the European Union after Brexit, keeping close ties to the single market and customs union in perpetuity, Labour’s Keir Starmer has insisted.

The Shadow Brexit Secretary made his remarks as the SNP leadership called for Labour to join it in a “united front” to keep Britain fully in the single market and customs union.

As MPs continue to debate the UK Government’s flagship EU Withdrawal Bill this week, the Liberal Democrats called on the Opposition to back its amendment to keep Britain in the single market.

Tom Brake for the Lib Dems said the Labour leadership's position on Brexit was as “clear as mud,” and that if Labour did not support its amendment it would “confirm suspicions Labour's leader and the Shadow Chancellor are in favour of a damaging hard Brexit".

But Sir Keir, stressing how his party wanted the UK to “stay aligned” with the EU, told the BBC's Andrew Marr Show: "We would start with viable options; staying in a customs union and a single market variant, which means full participation in the single market.

"You can't sweep the customs union and the single market off the table on the one hand and also say you don't want a hard border in Northern Ireland...You can't have no hard border, if you don't have alignment."

Asked if Theresa May's deal struck with the EU this week meant Britain would “in perpetuity” stay very close to the single market and the customs union, Sir Keir replied: "Yes.”

Asked if Britain would in such circumstances have to carry on paying money into EU coffers, he noted how Norway did so voluntarily, saying: “There may have to be payments; that's to be negotiated."

On freedom of movement, he said this too would have to be negotiated but agreed what Labour wanted was “easy movement” of workers on both sides of the Channel.

But David Davis claimed Sir Keir had shown the consequences of what Labour's “chaotic approach” to Brexit would be.

"A Labour Brexit would mean billions of pounds going to the EU in perpetuity, the UK being forced to obey rules over which we have no say and zero control over our borders. A Labour Brexit would betray voters and leave this country in the worst possible position,” declared the Brexit Secretary.

Describing the next round of talks on transition and trade as the “critical phase,” the SNP’s Ian Blackford called on Labour to “get behind us and others, who are arguing we should stay in the single market and the customs union”.

The Highland MP warned if Britain went into a hard Brexit without the protection of the single market and the customs union, there would be an “economic threat to this country, which is unparalled in recent times”.

Tomorrow, John Swinney, the Deputy First Minister, and Michael Russell, the Scottish Government’s Brexit Minister, are due in London for more talks with Damian Green, the First Secretary of State, and David Mundell, the Scottish Secretary, to discuss the EU Withdrawal Bill and the SNP administration’s objection to what it has labelled Whitehall’s “power-grab”. Failure to recommend Holyrood give its consent to the legislation would cause a constitutional crisis and hamper the Brexit process.

Meanwhile, underlying the tensions around Brexit, it was claimed the Prime Minister had to intervene in a heated dispute over planned defence cuts between Chancellor Philip Hammond and Defence Secretary Gavin Williamson, who supposedly became embroiled in a verbal disagreement in the Commons last week.

A source who saw Mrs May’s intervention, said: "She made it clear the two of them should grow up and calm down."

Mr Williamson also reportedly complained an ally of the Chancellor had compared him to Private Pike, the naive character in Dad’s Army.