He was last heard from as an opposition MP on 11 July 1996, in a debate on the civil service.
The minister yesterday came under fire from a new Liberal Democrat MP Norman Baker, who has tried - with little success - to call him to account through the parliamentary device of written questions.
A typical question last week asked what percentage of Mr Mandelson's working time was spent on ministerial duty and how much on "other matters." The minister replied : "I devote whatever time I judge necessary to the fulfilment of my ministerial and other duties."
Mr Baker, MP for Lewes, said the Government's new Code of Conduct for Ministers stated that ministers should be "as open as possible with Parliament and the public".
"The Minister without Portfolio is becoming the minister without accountability," he said. "I hope Peter 'the monk' Mandelson's own apparent vow of silence to the House will be broken by the new Code of Conduct next session."
Mr Mandelson said last night that as a minister without portfolio he could not be questioned in Parliament. But he would be able to answer questions on his specific ministerial responsibility for the Millennium Experience after the Commons returned from its summer break at the end of October.
"If Mr Baker wants to spend tens of thousands of pounds of taxpayers' money delighting himself with rather pointless questions, that is a matter for his judgement and his constituents. It is not a matter for me to comment on, and I would not do so."