The postal service in an independent Scotland may need to increase charges for customers or rely on government subsidies, the Tories have warned.

Conservatives fear SNP plans to renationalise the Royal Mail if there is a Yes vote in September's independence referendum could lead to higher costs and a poorer service.

First Minister Alex Salmond has already vowed that if Scotland leaves the UK and he is in power he will ''bring the Royal Mail, our postal service, back into public ownership''.

The Tory-Liberal Democrat coalition at Westminster has come under attack over its privatisation of the Royal Mail, with Labour leader Ed Miliband recently branding Prime Minister David Cameron ''the dunce of Downing Street'' for selling a majority of the company for £1.4 billion less than its current valuation.

Now Conservative MSP Murdo Fraser has hit out at the Scottish Government over its plans to bring the service north of the border back into public ownership in the event of independence.

He claimed: "Under the current set-up, Scottish businesses and residents enjoy being part of a larger body of consumers across the UK, which keeps prices down for everyone.

"Any move towards a separate Scottish postal service would create inequities that could result in higher delivery costs and potentially poorer service."

But a spokeswoman for Enterprise Minister Fergus Ewing said that was a "ridiculous claim", and added that the Tories' "reckless privatisation of the Royal Mail against the wishes of Scotland and of Scottish MPs has seen 1,300 redundancies announced and stamp prices increase for the first time in two years".

She added: "The only people who have benefited from privatisation are those in the City of London who made profits from the cut-price sell-off. Independence will deliver a Royal Mail in Scotland, in public hands that protects the universal services our communities rely on and keeps stamp prices down."

The Scottish Government's White Paper on independence pledges that postal services "will maintain at least the level of service provision inherited from the United Kingdom".

Mr Fraser has pressed the Scottish Government on the issue, asking a series of written parliamentary questions about the cost and timescale of renationalisation, and how the universal service obligation - which requires the Royal Mail to provide postal services at an affordable, uniform tariff across the UK - would be maintained.

Finance Secretary John Swinney said the "intention" was that "postal charges to the rest of the UK will not be more expensive than charges to send post within Scotland".

He said: "On independence, stamp prices will be the same as they are at the time in the rest of the UK. A Scottish postal service in public ownership would not need to generate profits for shareholders and so should be in a better position to ensure that postal prices and deliveries meet Scotland's needs. This applies to sending post and parcels within Scotland, to the rest of the UK and to other countries."

He vowed that the Scottish Government would "look to work with the remainder of the UK to ensure continuity of postal services" if Scots vote for independence.

Mr Swinney said: "Our approach to bringing the Royal Mail in Scotland back into public ownership will be considered in the light of circumstances at the point of independence, including the prevailing structure of the Royal Mail. It will require negotiation with the UK on Scotland's share of the government stake and establishing a new publicly-owned postal service in Scotland."

But Mr Fraser claimed the answers from the Scottish Government "have been light on detail but heavy on assertion, bluff and bluster".

The Conservative insisted: "What is clear is that a separate Scottish postal service can only operate with higher costs than at present. This requires either government subsidy, or increased charges.

"Alex Salmond must release a credible plan of how the Scottish Government might renationalise Royal Mail and maintain the same levels of postal service currently enjoyed by customers in Scotland. And Scottish businesses need to be aware of the threat of Salmond's stamp tax."