The Royal Mail should be renationalised in an independent Scotland but it is impossible to know how much this will cost, according to the SNP's postal affairs spokesman.

Mike Weir backed First Minister Alex Salmond's call for a moratorium on the UK Government's sell-off of Royal Mail until after the independence vote to protect assets which may transfer to Scottish public ownership.

He said a Scottish Royal Mail "would cost no more than it does at present", but said "nobody can know" how much it would cost to buy back the assets if UK privatisation proceeds as planned.

Mr Weir said: "We are asking the UK Government to delay the sell-off of a massive public asset that is vital to the economy, particularly in rural Scotland. We are a year away from making an important decision on our future, and we believe Royal Mail is essential to the wellbeing of Scotland."

He said Scotland should renationalise the Royal Mail if it gains independence, but added: "Nobody can know how much it's going to cost because we don't know how much is going to be sold off, we don't know the price, we don't know the value of that part of the business in Scotland at the time of privatisation, and the value of shares can go down as well as up."

Shadow postal affairs minister Ian Murray said: "I'm delighted that the First Minister has finally come on board, but he is seeing it through the prism of his obsession with the constitution rather than actually seeing this as an issue where we should be jointly fighting privatisation."