Addressing the Labour conference in Brighton today, Mr Gray is to throw down the gauntlet to Mr Salmond and challenge him to a head-to-head debate on Scotland’s future on St Andrew’s Day.

It is Labour’s second attempt in a week to bait Mr Salmond. Last Monday, he dismissed a similar invitation from Jim Murphy, the Scottish Secretary, on the grounds he debated, and bested, Labour’s Holyrood leader each week.

“Alex Salmond has refused to debate with the Scottish Secretary but he cannot run and hide from his direct opposite number at Holyrood,” said Mr Gray.

“If Mr Salmond has any faith at all in his plans he will accept a debate with me on his disastrous plans for separation.”

The symbolism of the St Andrew’s Day challenge is that it is the preferred date for the SNP government to introduce its referendum bill to the Scottish parliament.

Mr Gray, who has served just more than a year as Labour’s Holyrood leader, has still to dent the First Minister’s armour. Today he will use a morning speech at the conference to renew his attack on the SNP government. “I know what my vision for Scotland is. I want our children to grow up in a prosperous country that believes in equality and social justice. Being part of the world’s most successful political union enables us to achieve this,” said Mr Gray. “I am prepared to defend my political values against Alex Salmond and meet him head on in a debate on Scotland’s national day. Let him explain why he is lining up with the Tories to put David Cameron into Downing Street.”

Both Mr Gray and Scottish Secretary Jim Murphy will address the conference consecutively in what Labour officials described as a “beefed up” Scottish report that

will introduce Glasgow

North East candidate Willie Bain to delegates. Emphasising the joint approach, Mr Gray will go for the SNP while Mr Murphy will warn of the dangers of the Tory threat to Scotland.

Mr Murphy will say the choice is between Tory and Labour and appeal to other party supporters to make a call. Labour officials claim private polling shows nine out of 10 SNP supporters would prefer a Labour government to a Tory one.

Mr Murphy is expected to say the Tory candidates are among the most hardline in memory, dubbing them “Thatcher’s grandchildren”.

“In Scotland, David Cameron is even less popular today than Mrs Thatcher was in the 1980s -- but he is no less a threat to Scotland’s families and our economy.

“And the current crop of Scottish Tory candidates are probably the most hardline in living memory.”

An SNP spokesman asked what Mr Gray hoped to achieve in a set-piece debate “that he cannot try to achieve at First Minister’s Questions each week?”

He added: “The First Minister’s sights are set on a UK General Election debate with the Prime Minister.”