ALEX Salmond is facing calls to apologise after accusing his opponents of using immigration as a "weapon" in the referendum campaign.

His comments, in an article in The Herald yesterday, provoked an angry backlash from Labour, the Conservatives and Lib Dems.

Willie Rennie, the leader of the Scottish Liberal Democrats, said Mr Salmond has made an "irrational outburst" after claiming Holyrood's three main opposition parties were copying Ukip by "engendering fear" of immigration. He added: "To accuse Scotland's liberal party of Ukip-like extremism on immigration is contrary to reality.

"We stood up to Farage when others shied away. Alex Salmond is resorting to such over-blown remarks to cover the weakness of his independence plans. It should be beneath any First Minister to use such aggressive militaristic language, like 'weapon' when discussing an issue that deserves great sensitivity."

He said: "The First Minister should apologise for seeking to divide rather than unite on immigration." Mr Salmond's comments came in a article written to coincide with a speech on immigration at Dundee University last night.

Last week he confirmed an SNP Government in an independent Scotland would seek to attract an extra 24,000 people per year to live and work in the country as part of a package of measures to increase tax revenues by £5billion by 2030.

He argued the figure was 2,000 more than Scotland's average annual net migration of 22,000 between 2001 and 2011.

However, the period saw relatively high levels of immigration from new EU countries such as Poland. The UK Treasury estimates Scotland's average annual net migration will be 15,500 over the next 20 years.

The issue of immigration has not been centre stage in the independence debate up to know, though Yvette Cooper, Labour's Shadow Home Secretary, said in April that an independent Scotland would need nearly a million migrants by 2051 to fund pensions.

She also urged Mr Salmond to "come clean" about his policies.

In the article Mr Salmond said: "The main Westminster parties have decided to deploy immigration as a weapon in their increasingly tawdry self-styled Project Fear campaign."

Dismissing the claim, former Labour Prime Minister Gordon Brown said: "We don't agree with Ukip and Alex Salmond is trying to equate everyone with Ukip. Everybody knows we are totally opposed the politics of Ukip."

Scots Labour leader Johann Lamont added: "I think it speaks more about Alex Salmond than it does about the big issues we are wrestling with in terms of why people supported Ukip."

Scottish Conservative MSP Alex Johnstone said immigration concerns in Scotland were roughly the same as in England but the SNP wanted to make it into an issue.

A spokesman for Mr Salmond said the First Minister's comments "spoke for themselves".