SNP MPs will vote down Theresa May's final Brexit deal in the House of Commons if it does not commit the UK to continued membership of the European single market, Ian Blackford has said.

Speaking ahead of party conference, the SNP's Westminster leader also said his MPs would be open to discussions about a second EU referendum - the so-called "People's Vote".

He was echoing comments earlier by Nicola Sturgeon, who said SNP MPs would "undoubtedly" back a second Brexit vote if it was put before the Commons.

However, Blackford also said the Scottish Government has a mandate to hold a second Scottish independence referendum.

Speaking to STV News, he set out the party's "red line" for whatever Brexit deal the Prime Minister returns to Parliament with from Brussels - that Scotland is kept in the single market and customs union.

SNP MPs would refuse to accept a deal that made people in Scotland poorer, he added.

"There is a real threat to the economy if we're not inside the single market and customs union," Blackford told STV.

"A red line for us is if Theresa May comes back with a plan that is going to result in job losses for the people of Scotland then, of course, we have to reject that.

"If she wants to make sure that she can protect jobs in Scotland as possible, we have to stay in the single market, we have to stay in the customs union.

"If she can't deliver that, we'll be voting against her Brexit deal."

He added: "If we are going to be dragged out of the single market and customs union against our will, we need to look at the other options that we have at that point - discussions about having a People's Vote.

"But I think more importantly, we have to recognise the Scottish Government does have a mandate, if the circumstances are right, that we can go back to the people of Scotland and ask the people of Scotland to make their own judgement about our constitutional future.

"We are a European nation, we wish to remain a European nation."

Pressed on if voting against May's plans could cause a no-deal Brexit, Blackford insisted it would not.

The Ross, Skye and Lochaber MP continued: "One of the things I want to make sure is we've got the opportunity to say to the European Union that there is a majority in the House of Commons to stay in the single market and customs union.

"I want that proposition to be brought forward.

"Every member of Parliament in the House of Commons has got to look at themselves at that point and say: 'What is in the best interests of our constituents? Am I prepared to end up with constituents being on the dole?'

"There's a collective responsibility that we all have, and I hope Parliament shows its teeth, and shows that we have to act in a responsible manner to protect the people of Scotland and, indeed in this case, of the United Kingdom."

The comments come after the Court of Session in Edinburgh recently fast-tracked to the European Court of Justice (ECJ) a decision about whether or not MPs can vote to halt the Brexit process.

It revolves around whether the House of Commons can unilaterally withdraw the UK Government's Article 50 letter to Brussels, which commits Britain to leaving the EU on March 29 next year.

The case was brought by a cross-party group of six Scottish MPs, MEPs and MSPs, including prominent SNP MP Joanna Cherry and Green MSP Andy Wightman.