But postal unions have already lambasted the proposal as allowing the private sector to cherry-pick the profitable parts of the service while leaving postal workers uncertain about their retirements.

Nigel Stapleton, chairman of the postal regulator, will argue that the Government should encourage private investment in the service by reassuring potential buyers they would not assume liability for its gaping pensions deficit. According to the last estimate, Royal Mail's pensions deficit stands at £3.4bn, but that figure is expected to be revised substantially upwards when the latest valuation is revealed next month. Some estimates suggest it may have more than doubled to £7bn.

Analysts say the pensions problem would make it extremely difficult for the Royal Mail to attract the private investment both Postcomm and its own management say it needs to modernise.

In an interview with Scotland on Sunday ahead of a visit to the Scottish Parliament this week, Stapleton said the pensions black hole was amassed during the 1980s and 1990s, and has nothing to do with its current operations. He said it is standard practice in the private sector when firms buy a stake in another firm that they do not automatically assume liability for historical problems.

The Royal Mail should not be an exception, he argued.

"Usually the new owner does not take on those liabilities," he said. "If the Government was to make the decision to offer a stake in the Royal Mail, then something would have to be done to make sure the owner does not take on those (pensions] liabilities."

Stapleton is seeking to widen support for the part privatisation of the Royal Mail after Postcomm recommended in May that it should be allowed to raise external capital – a model that has been tried and tested in several other European countries. In the same month, Royal Mail made its own plea to issue shares.

But unions last night dismissed Stapleton's proposal as a "horrific" example of allowing the private sector to invest in the profitable parts of the business, while leaving the state to struggle with its debts.

A spokeswoman for the Communication Workers Union said: "Pensions are a crucial part of workers' benefits. All companies, including Royal Mail, have an obligation to their workforce to uphold their agreed terms and conditions, especially pensions. We expect employers in the mail industry to provide decent pay and working conditions for postal workers and not use competition as an excuse to undercut their terms or reduce pension provision."

She claimed the reason that Royal Mail is labouring under such severe problems – it posted a loss of £279m in May – is due to Postcomm's decision to open the market up to competition ahead of the rest of Europe, and for setting "incredibly low" tariffs for the "final mile", the stage where post is delivered.

Private companies, such as TNT Post and Business Post, have set up profitable businesses collecting bulk mail from businesses, but use the Royal Mail for the final stage of delivery. Postcomm decides the tariffs for this final stage, and the CWU argues that the prices set are putting Royal Mail's competitors at an advantage.

But Stapleton will this week seek to persuade Scottish ministers and businesses that the liberalisation of the market is not the root cause of the postal service and Post Office network's demise.

He argued that the single biggest reason for the fall in mail volumes has been the digital revolution, as people switch to sending e-mails rather than letters.

"That is removing value from the Royal Mail to a greater extent than Royal Mail is losing value to new mail competitors," he said.

The Government is preparing to publish the results of the Hooper review into the future of Royal Mail in October.