Lord Mandelson says he has been "very open"
Lord Mandelson is facing renewed calls to explain dealings with Russian tycoon Oleg Deripaska after they fail to appear in his declaration of interests.
Liberal Democrat MP Norman Baker described the new business secretary's entry in the House of Lords' register as "threadbare and totally inadequate".
Lord Mandelson has insisted he has been open about visiting the billionaire's yacht while he was the EU trade chief.
But Mr Baker called for all the pair's meetings to be recorded publicly.
Lord Mandelson's first entry in the Lords' register since rejoining government last month lists his regular paid employment as his ministerial salary and a "transitional allowance" awarded him by Brussels as departing European commissioner.
He also entered details of his chairmanship of the Policy Network think tank, his honorary fellowship of St Catherine's College, Oxford, and his presidency of Hartlepool United Football Club - the town where he used to be MP.
'Location and purpose'
Mr Baker insisted more information needed to be made public.
"It is vital that we know who Lord Mandelson dealt with when he was European trade commissioner as he will be dealing with the same people in his role as business secretary," Mr Baker said.
He added that if Lord Mandelson was "genuinely committed to openness" he should publish his diary for his last year as European trade commissioner.
In these conversations with this gentleman, there was no discussion of EU trade business, no discussion about tariffs or anti-dumping duties
"This must include details of all meetings with Mr Oleg Deripaska and their location and purpose."
Earlier, Lord Mandelson insisted he had been "open" about his dealings with Mr Deripaska, an aluminium magnate, and did not face a conflict of interest at any stage.
The business secretary also said he had not discussed any issues relevant to the billionaire's commercial interests when he met him as EU trade commissioner.
His involvement with Mr Deripaska was "different" to that of shadow chancellor George Osborne, he argued.
Mr Osborne has said he made a mistake by meeting Mr Deripaska but has strongly denied allegations he tried to solicit a £50,000 donation for the Conservative Party from the tycoon during a visit to his yacht in Corfu in August.
The Conservatives have already urged Lord Mandelson to disclose information about any hospitality he received from the businessman in the Lords' register of members interests.
'No action needed'
Lord Mandelson, who is on a visit to the Gulf with the prime minister, has denied that he needs to take any "further action" to explain his conduct during meetings with Mr Deripaska since 2004.
"I think I have been very open," he told the BBC.
"The question is not how many times I talk to somebody but what happens in the conversations.
"In these conversations with this gentleman, there was no discussion of EU trade business, no discussion about tariffs or anti-dumping duties."
He added: "It is simply not possible for me as a commissioner in these circumstances to do favours for any individual and I would not even dream of trying even if I could."
The Electoral Commission and the Parliamentary Standards Commissioner have rejected calls from Lib Dem and Labour MPs to investigate Mr Osborne's conduct, saying there was no evidence he committed any offence.