According to legal advice from the House of Commons Library, an organisation that interprets legislation, any disposal of shares that is not into a joint venture would require new primary legislation.

The advice was given to John Grogan, the Labour MP for Selby, who has secured the support of 171 MPs for an early day motion, insisting that Royal Mail remains a full public entity with no share offers to employees.

Mr Grogan’s supporters include 162 Labour MPs — a clear show of disapproval for the plan, which is being considered by Alan Johnson, the Trade and Industry Secretary.

The plan has been championed by Allan Leighton, the chairman of Royal Mail.

However, the Treasury is understood to be concerned about the cost of administering the scheme and is also thought to be sceptical about its merits.

The Government is also considering requests from Royal Mail for additional funding but the Treasury is reluctant to take on the organisation’s £2 billion of pension liabilities.

That uncertainty has led Sir George Bain, who is advising the Government on Royal Mail’s future, to put off any recommendations that he was expected to make in an interim report due to be delivered to Mr Johnson today.

Sir George is likely to make recommendations in December at the earliest. He also wants to wait until after the industry regulator has made a decision on pricing, which is not expected until next month.

Mr Grogan said that a share offer to staff should be resisted because it was likely to lead to full privatisation. He said: “Alan Johnson asked Labour backbenchers: ‘Why not employee ownership?’ but we would ask: ‘Why?’ It would be a cost to the business and would not bring in additional funds.

“The real motivation must be a softener for privatisation and there would seem to be no doubt that Allan Leighton wants that.”

Billy Hayes, general secretary of the Communication Workers Union, said: “Any issue of shares would be a betrayal of the Government’s election manifesto. There is no support for a phoney share issue anywhere.”

Royal Mail declined to comment but executives are thought to believe that if new legislation is needed then it could be rushed through Parliament. Royal Mail is keen to change its structure in time for the opening of the market to full competition in January.

Should political opposition continue, Royal Mail managers could ballot the 180,000 staff on whether they want shares.

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