A hard Brexit would make Scottish independence much more difficult to achieve, a leading academic has argued, as the political war of words over retaining membership of the European single market intensified.

Michael Keating, Professor of Politics at the University of Aberdeen, told MPs that if Scotland were in the EU and the rest of the UK were not, then that would break up the single British market between them, creating a hard border; an economic barrier to free movement of goods, people and services between the two countries.

READ MORE: Alex Salmond: Single market could bring 'early' independence referendum

Asked how Scotland would be able to stay within the UK single market - which was worth four times more to the Scottish economy than the European single market, - if it became independent, Prof Keating told the Commons Scottish Affairs committee: “They don’t.

“If it is a hard Brexit and the UK comes out of the European single market, then it makes it much more difficult for Scotland to become independent because you’d have that hard border with the UK market.”

He then added: “Ironically enough, the closer the UK gets to the single market, then the less grievance Scotland maybe has but the more easy it is to become independent because at that point you could have access to both markets.”

Ian Murray, Scottish Labour’s Westminster spokesman and a committee member, said: “What Prof Keating is suggesting is that it is in Scotland’s best interests to stay within the UK single market, which is the most important market for Scotland, and for there not be a hard border but for the UK Government to negotiate the best deal possible so as to have as much access as possible to the European single market.”

In further evidence to MPs, Jo Murkens, associate professor of law at the London School of Economics, suggested while the Scottish Government did not have a constitutional veto over the Brexit process, it did have an “effective” political one.

READ MORE: Alex Salmond: Single market could bring 'early' independence referendum

“The role of Scotland is not just to say Yes to any deal hammered out by Westminster,” declared Dr Murkens.

At Holyrood, Nicola Sturgeon was challenged to set out the terms of the UK Brexit deal that would persuade her to abandon any plan for a second independence referendum.

Ruth Davidson, the Scottish Tory leader, told MSPs: "The First Minister declares that independence will be considered 'only if it is best or only way to protect our membership in the EU'.

"But can the First Minister honestly tell the chamber, under what circumstances and on what issue she has ever concluded that it isn't the best option for Scotland?”

Ms Sturgeon has set out an ambitious wish-list of five key demands she would like to see in any post-Brexit deal, including membership of the single market.

She said: "To give up the right to even consider that option [of independence] would be to accept we are at the mercy of Westminster decisions no matter how damaging or destructive they are to our society, our economy or place in the world."

At Westminster, Brexit tensions continued with Theresa May insisting she would not give a running commentary on her position on leaving the EU after slapping down David Davis, the Brexit secretary, who this week told MPs it was “very improbable” the UK would retain membership of the single market if it meant losing control of its borders.

Angus Robertson, the SNP leader in the Commons, accused the Prime Minister of peddling “waffle” while Tim Farron, the Liberal Democrat leader, said the lack of detail showed the Tory Government did not have a clue.

READ MORE: Alex Salmond: Single market could bring 'early' independence referendum

Elsewhere, Alex Salmond, the former First Minister, told his weekly radio phone-in show that if there were no compromise or ability to keep Scotland within the single market as part of the UK, then “that will mean an early[independence] referendum; if there is, the referendum might be later".