He has insisted he has not broken the ministerial code of conduct by not registering his loan of £373,000 from Geoffrey Robinson.

Bill Bush, head of BBC Research: "He has never registered the loan as an interest"

When Labour came to power last year, Tony Blair issued a revised Code of Conduct and Guidance on Procedures for Ministers, tightening up the rules and implying that his ministers would have stronger principles than the previous government.

The code warns ministers that they must "scrupulously avoid any danger of an actual or apparent conflict of interest between their ministerial position and their private financial interests".

It states: "It is a well established and recognised rule that no minister or public servant should accept gifts, hospitality or services from anyone, which would, or might appear to, place him or her under an obligation."

The code explicitly bars ministers keeping any gift worth more than £140.

Mr Mandelson has also had to defend not entering the loan in the Register of Member's Interests, where all MPs must register income that could influence their action in parliament.

The Register of Member's Interests

The register was established in 1974 although its form was changed when the Committee for Standards in Public Life was set up 1995 after the "cash for questions" scandal.

Public standards committee member Anthony King: "The rules are pretty tight"

The committee, chaired by Lord Nolan, tightened the regulations on what information MPs are required to submit to the register.

MPs are sent forms with 10 sections to complete in their own words and return within three months of being elected. They are then expected to keep the register up to date.

According to the Parliamentary Commissioner for Standards, Sir Gordon Downey, its aim is to: "Provide information of any pecuniary interest or other material benefit which a member receives which might reasonably be thought by others to influence his or her actions, speeches or votes in Parliament, or action taken in his or her capacity as a Member of Parliament."

Mr Mandelson was lent £373,000

It later states in the category of miscellaneous and unremunerated interests: "This is a discretionary section for use by members wishing to register interests, including unremunerated interests, which do not clearly fall within any of the specific categories but which they consider to be relevant to the definition of the register's purpose."

In the latest version Peter Mandelson, like many senior ministers who give up outside interests when they take their place in government, lists nothing.

In 1996, the year of the loan, Mr Mandelson's registered interests including fees from journalism, broadcasting, speaking and membership of the Harris Parliamentary Panel.

He also lists several business visits for the year, including trips to Hong Kong, New York and Spain.

Mr Blair's spokesman said Mr Mandelson would be writing to the new Registrar of Members Interests, Elizabeth Filkin, setting out details of the loan, for her to decide whether he had breached parliamentary rules by not declaring it.