Labour has toughened its stance on Brexit after days of confusion, siding with the SNP by demanding a special deal for Scotland.

Sir Keir Starmer, the party’s spokesman on leaving the EU, also said Britain must stay in the customs union – which would rule out any separate trade deals with non-EU countries.

However, Sir Keir also went further in insisting Labour will not block the triggering of the Article 50 notice, even if the Government refuses its demands.

Labour has been under fire after Jeremy Corbyn’s announcement of ‘Brexit bottom lines’ – apparently threatening to hold up Article 50 - was contradicted by his deputy, Tom Watson, within 24 hours.

The party leader said he wanted tariff-free access to the single market, no watering down of workers' rights, protections for consumers and the environment, and a pledge to match lost EU funding.

Amid alarm that Mr Corbyn had gone too far, Mr Watson then insisted: “We are not going to hold this up. The British people have spoken and Article 50 will be triggered when it comes to Westminster."

Nevertheless, Sir Keir has gone further in setting out Labour’s demands, in the Bill that is likely to follow the High Court ruling that Theresa May cannot bypass Parliament when she triggers withdrawal.

Corbyn insists parliament should not vote on Article 50

He told the BBC’s Victoria Derbyshire programme: “If it is legislation, then of course there might be amendments put down.

“We are clear that we need the fullest possible access to the single market, that we should be in the customs union, and that there should be special arrangements for Northern Ireland, Wales and Scotland.”

Most business leaders are desperate to remain within the customs union, which allows exporters to sell into the single market without having to fill in forms or go through customs checks.

But, while staying in would cut costs and red tape, it would prevent Britain striking separate trade deals such as the one the Prime Minister is pursuing in India this week.

The stance on the customs union opens the door for a future alliance with Conservative MPs who favour a ‘soft’ Brexit, by pushing an amendment in the Commons.

The SNP – Labour’s traditional enemies in Scotland – have, under Nicola Sturgeon, led demands for a special Brexit deal, after a big majority of Scots voted Remain.

Ms May has promised to involve Scotland, Wales and Northern Ireland in regular formal talks on the Brexit process - but she has ruled out allowing special deals for the devolved nations.

However – attempting to draw a line under the confusion about Labour’s position – Sir Keir said there were no circumstances in which it would vote against invoking Article 50.

He said: “We are not going to frustrate the process.” Asked if that mean no circumstances in which Labour would block it, he replied: “Yes”.

In the interview, Sir Keir said he accepted “changes to the way freedom of movement rules operate have to be part of the negotiation”.