ALEX Salmond has been called on by Labour to reveal any studies of the impact of independence on food prices after some supermarket chiefs suggested breaking away from Britain could cost Scottish shoppers.

Margaret Curran, the Shadow Scottish Secretary, said the intervention by three supermarket bosses was a "devastating blow" to the case for independence but Scottish Rural Affairs Secretary Richard Lochhead insisted Tesco had made it clear it had made no such comments while Asda and Morrisons stressed they had no plans to up prices.

Tesco is Scotland's biggest private sector employer with 31,500 staff, Asda is second biggest, with 19,500. Fifth largest Morrisons has 12,000.

An unnamed top executive was quoted was saying if Scotland gained independence: "We would treat it as an international market and act accordingly by putting up our prices. The costs of distribution are much higher in Scotland but at the moment that gets absorbed by the UK business."

Asda chief executive Andy Clarke said: "A Yes vote in 2014 could result in Scotland being a less attractive investment proposition for business and put further pressure on our costs."

Morrisons' boss Dalton Philips said:"Why should the English and Welsh consumer subsidise this increased cost of doing business in Scotland?"

Ms Curran said: "The message from supermarket bosses is clear: The cost of doing the weekly shop in Scotland is cheaper as part of the UK."

Mr Lochhead said: "The No campaign's claims have completely backfired as Asda have also made clear their comments also apply to the powers devolved to Scotland in the 2012 Scotland Act; something brought in by the Westminster parties in advance of independence."