It was claimed during a High Court libel action on Monday that in 1990, when Mr Mellor was Minister for the Arts, he accepted a gift of air tickets for himself and his family to go on holiday to Marbella with a freelance film and theatre producer, Mona Bauwens.
Under Whitehall's ministerial rule-book, Questions of Procedure for Ministers, it is laid down as 'a well-established and recognised rule that no minister . . . should accept gifts, hospitality or services from anyone which would, or might appear to, place him or her under an obligation.
'The same principle applies if gifts, etc., are offered to a member of their family.'
Bryan Gould, Labour spokesman for National Heritage, said yesterday: 'It does seem to me that there was a breach of the rules.' Commending John Major's decision to publish the previously-confidential ministerial rules last May, he added: 'Now is his chance to show that he intends to enforce them. He should enquire if there has been a breach, and, if so, he should reprimand Mr Mellor at the very least.'
Mr Gould said: 'Mr Mellor's capacity to act effectively is steadily being eroded and it is going to get more difficult for him. I believe he will either go, or something will change, within a relatively short period.'
Although the ministerial rules state that ministers should not accept gifts at all, they do suggest that 'in doubt or difficulty' the Prime Minister's guidance should be sought.
Asked whether Mr Mellor had sought Margaret Thatcher's advice over the air tickets for himself, his wife and two children in 1990, No 10 Downing Street said last night: 'Records aren't kept of this sort of thing.'
The ministerial rules also state that 'in all cases' where a gift has to be accepted, where, for example, refusal might 'risk apparent discourtesy' to an overseas government, it must be reported to the Permanent Secretary of the minister's department.
Asked whether Mr Mellor had reported the gift to the Cabinet Office, which covered his responsibilty as arts minister, a spokeswoman referred the Independent to his current department. A National Heritage spokesman said: 'These are matters for ministers and the department.' The Cabinet Office said later: 'We are not going to add anything to what they have said.'
Ministerial rules state that when ministers have to accept gifts from foreign governments, they 'may be retained' if they are worth less than pounds 125. It is estimated that one adult return to Malaga in 1990 would have cost about pounds 159.
Under Commons procedures, MPs are required to declare 'any payments or any material benefits or advantages received from or on behalf of foreign Governments, organisations or persons'. It is not known whether Mrs Bauwens is a foreign national, but there is no record of a Commons declaration by Mr Mellor of the 1990 tickets. .
As the People began its defence on the second day of the libel action brought by Mrs Bauwens, Richard Stott, editor at the time of the articles in August 1990, said the criticism was of Mr Mellor's judgement in remaining in Marbella, having gone there in the first place on a 'freebie holiday'.
He said: 'We wanted to point out that Mr Mellor's political judgement was extremely flawed, in our view. It was hugely in the public interest that it should be reported. The alternative to printing it would be to note it and suppress it.'
He said he would have used the story irrespective of whether it had been a Labour or Tory minister involved.
Libel hearing, page 3
Leading article, page 24