One million Scottish jobs are currently connected to the country's position as part of the United Kingdom, Gordon Brown has warned as he faced criticism from Labour MPs over his recent interventions in the 'No' campaign.

In a major speech in London he unveiled new figures that he said showed almost 40% of Scottish jobs were either directly or indirectly linked to the Union. But the former Labour Prime Minister's call came amid growing anger from Labour politicians over his recent comments in the debate.

Earlier this week Mr Brown criticised parts of the pro-Union campaign, warning that it risked patronising Scots.

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There was considerable surprise when he broke ranks to call on the Conservative leader David Cameron to take part in a televised head-to-head debate with Alex Salmond, something the SNP leader has been demanding for months.

Mr Brown's remarks were seen as potentially overshadowing the Better Together leader, and his former Chancellor, Alistair Darling, who has challenged Mr Salmond to a separate showdown.

One Scottish Labour politician said that he had "never seen" so many of his colleagues so angry at Mr Brown. Another predicted that the fallout would cause "irreparable damage" to the former Labour leader's relationship with many Scottish Labour MPs.

In a speech at the London School of Economics Mr Brown said that 40% of all Scottish jobs were linked, directly or indirectly, to the UK.

He predicted that Scotland would see more investment from countries like China if it remained within the Union.

Mr Brown said: "Almost one million jobs are linked to our membership of the UK.

"The exact total of 962,000 jobs includes 313,000 Scottish jobs in companies controlled from the rest of the UK and 247,000 Scottish jobs in companies exporting from Scotland to the rest of the UK."

The "conservative and robust" figures have been calculated by Professor Brian Ashcroft at the Fraser of Allander Institute at Strathclyde University.

Taking questions after his speech, Mr Brown insisted that he had not called on Mr Cameron to debate the First Minister.

He said: "I was asked if I was PM would I debate. Of course I would debate. That is what I said."

He said he had been talking about the "hundreds of debates" that are taking place across Scotland, adding "the main debates are going to be between Alistair Darling, the leader of the Better Together campaign, who is doing a very good job, and Alex Salmond".

Last night the SNP accused Mr Brown of making "ridiculous" claims about the number of jobs linked to being part of the UK.

SNP MSP Kenneth Gibson said: "These are ridiculous claims from Gordon Brown - after a Yes vote Scotland will continue to have a close and positive relationship with our neighbours, on an open-border basis, and the rest of the UK will remain our biggest trading partner. On Mr Brown's absurd logic, Canada should join with the United States and be governed from Washington."

He added: "Just yesterday, Gordon Brown said that the No campaign was too negative - today he is making scaremongering claims which don't stand up to a second's scrutiny. He is clearly a fully paid-up member of the No campaign's Project Fear."

l Former SNP deputy leader Jim Sillars has called on the UK Government to bring a Bill at Westminster legislating for the extra powers the pro-Union parties have promised Scotland after a No vote in September.