Failure to keep Scotland within the European single market as part of the UK will result in an "early" second referendum on independence, according to Alex Salmond.

The former first minister said membership of the free trade zone would affect the timing of a second ballot rather than whether one takes place at all.

During his phone-in programme on LBC radio, Mr Salmond was pressed on whether the single market was a "red line" issue or whether he would still want to see a second independence referendum within the next few years regardless of the UK's membership status.

He said: "The SNP is a pro-independence party but Nicola Sturgeon has identified that our key priority is to keep Scotland within that single market place.

"I think she is right to prioritise that."

He added: "I think that (single market membership) would affect the timing as opposed to whether there was a referendum, because I think if there is no compromise or no ability to keep Scotland within the single market within the context of the UK, that will mean an early referendum - if there is, the referendum might be later."

Mr Salmond's remarks followed Theresa May's earlier refusal to say whether she wants the UK to stay in the single market after it leaves the European Union, telling MPs she will not reveal her Brexit negotiating hand ''prematurely''.

At Holyrood, Ms Sturgeon dismissed Scottish Labour leader Kezia Dugdale's suggestion there had been a recent ''shift'' in the First Minister's approach, from a focus on retaining Scotland's EU membership to only seeking access to the single market.

Ms Sturgeon said: ''I've said all along that I will examine all options to protect Scotland's interests.

"There is no doubt that what I see as the best option is to retain our membership of the European Union and I will be working to seek to do that.

''But I'll also be working along the way to try to protect all the aspects of EU membership that we possibly can.''