An independent Scotland will have the opportunity to keep its universal postal service, the First Minister said amid fears it could be under threat at the recently privatised Royal Mail.

Royal Mail's warning that competition from rivals "could threaten the fundamental economics of the universal service" is "deeply concerning", Alex Salmond said during First Minister's Questions.

He urged the UK Cabinet's embattled Liberal Democrat ministers to protect the universal service obligation (USO) which requires Royal Mail to deliver across the UK for the same price.

The USO is a "lifeline" for rural communities such as Lib Dem Chief Secretary to the Treasury Danny Alexander's Highland constituency, Mr Salmond said.

Mr Alexander is deeply unpopular in his Inverness, Nairn, Badenoch and Strathspey seat, a leaked poll by former Lib Dem peer Lord Oakeshott suggested this week.

Lord Oakshott's friend Vince Cable, the UK Business Secretary, has faced questions over whether he knew about the poll which also suggested Deputy Prime Minister Nick Clegg could lose his seat at the next election.

SNP MSP Dave Thompson, whose Holyrood seat sits within Mr Alexander's Westminster constituency, questioned whether the Lib Dems can be trusted with the Royal Mail.

He said: "As recently as last month, Vince Cable said: 'The sale of shares in Royal Mail has delivered on our commitment to protect the universal postal service and safeguard vital services for the taxpayer.'

"My local MP Danny Alexander also once said: 'We must continue to be vigilant and safeguard the USO at all costs.'

"Does the First Minister believe that these Lib Dem MPs are to be trusted?"

Mr Salmond said: "It will be speculated on over who commissioned the poll. Did Vince Cable know about it? The only thing certain is that we know the results.

"I believe this undermining - or let's call it questioning or a concern - which has been said by the Royal Mail executives about the USO brings very carefully into the front-line of public discussion something that those of us who opposed the privatisation of Royal Mail were deeply concerned about.

"We will be looking for more assurances that this USO guarantee still stands.

"Whether we get them from Danny Alexander or Vince Cable, I suspect we should get them quickly or we might not get them at all."

Last week, Royal Mail chief executive Moya Greene said rivals' ability to "cherry-pick" easy-to-serve urban areas and leave the less economical addresses to Royal Mail "could threaten the fundamental economics of the universal service".

Mr Salmond added: "It's deeply concerning the see the Royal Mail's concerns about its ability to fulfil the USO.

"It is disgraceful that this public asset was sold at a knock-down price.

"Regulation of mail in the hands of the Scottish Parliament will provide us with the opportunity to ensure that universal postal service is there in the best interests of communities across this country."