A second vote on Scottish independence could take place in the autumn of 2018, former first minister Alex Salmond has said.

The ex-SNP leader made the suggestion after his successor Nicola Sturgeon warned the Prime Minister's plan to take the UK out of the European single market ''undoubtedly'' makes another vote on the future of the UK more likely.

Mr Salmond said such a vote could take place in the autumn of next year - four years on from the September 2014 referendum in which Scots voted by 55% to 45% to stay part of the United Kingdom.

With the Scottish Government having repeatedly made plain its desire to keep Scotland in the single market, Mr Salmond said that if Theresa May "flings down the gauntlet" and proceeds with her plans for a so-called hard Brexit, then "Nicola Sturgeon will pick it up".

During his LBC radio phone-in show, he added: "The ball right now is in Downing Street's court."

Pressed on the timing of a possible second referendum, he said it would have to take place during the two-year Brexit negotiating period, which will begin in March when Mrs May triggers Article 50.

Mr Salmond said: "It has to be within the two-year negotiating period, so we would be talking about a year come the autumn, or something like that.

"You would have to have it within the two years, you would want to have the Scottish referendum before the Brexit process had been completed."

In a speech on Tuesday setting out her 12 key objectives for EU withdrawal, Mrs May announced Britain will leave the European single market but will seek a ''bold and ambitious'' free trade agreement to allow it to continue doing business with its 27 former partners without having to pay ''huge sums'' into EU budgets.

The Prime Minister also confirmed she wants to take Britain out of the jurisdiction of the European Court of Justice and restore control over immigration.