Brexit-backing ministers complaining about restricted access to Government papers should consider themselves lucky not to be facing the sack, former business secretary Lord Mandelson said.
His comments came as it emerged Leave-supporting Cabinet ministers could be forced to use the Freedom of Information (FoI) Act to gain access to official documents from their own departments relating to the EU referendum.
The top civil servant in the department of Leave campaigner and Northern Ireland Secretary Theresa Villiers told MPs that, under the terms of the guidance issued by Cabinet Secretary Sir Jeremy Heywood, the minister might have to use a FoI request or a parliamentary question to obtain information produced by her own staff.
Lord Mandelson said pro-Brexit ministers should stop "whinging" about the situation and suggested the only paper they should expect is a P45.
David Cameron has allowed members of the Government to campaign for a Leave vote even though the executive's official position is for the UK to remain in the European Union.
But pro-Brexit ministers will not be allowed papers relating to the referendum question under Sir Jeremy's guidance.
Intervening in the row over the contents of "Chris Grayling's in-tray", Lord Mandelson said: "Frankly, I think, these complaining ministers are lucky. Usually when members of a government go against ministerial collective responsibility and the will of the Cabinet, they receive one paper - and that's their P45.
"So I think they have got off rather lightly and they should stop whinging."
Lord Mandelson also singled out Boris Johnson for criticism following the London mayor's comparison of himself to James Bond, standing up for Britain against the forces of Brussels.
The Labour peer said: "It would be thoroughly reckless for Britain to sacrifice the settled network of trade advantages and preferences that we have built up over decades through our membership of the EU, all for the thrill of a daredevil race around the international circuit in our own Aston Martin as Boris had it over the weekend."
Northern Ireland Office permanent secretary Sir Jonathan Stephens was challenged over Ms Villiers' access to papers as he appeared before the Public Administration and Constitutional Affairs Select Committee.
Asked whether he would withhold from Ms Villiers information bearing on the referendum which 10 Downing Street or the Cabinet Office had asked her department to produce, he said: "Yes. That information is to be used to support the policy of the Government of the day."
Ministers campaigning for the UK to leave the EU are "operating in a personal capacity and in that respect, if they put in a freedom of information request or a parliamentary question, that will be answered, but they won't be receiving the support of the Civil Service", Sir Jonathan told the MPs.
He confirmed that NIO civil servants would continue to check facts relating to the EU referendum for Ms Villiers, and said the Cabinet Secretary would ultimately be the judge of what specific material she could see.
Sir Jonathan insisted the guidance was not having "any impact on the normal functioning of the department" and said he was "confident we can make the arrangements work".
He said he had discussed Sir Jeremy's instruction with Ms Villiers, who made clear she supports the arrangement and "understands that that is the position the department will need to adopt and that it needs to operate in support of Government policy".
The main focus of Lord Mandelson's speech in the City of London was the impact of Brexit on the UK's trading links.
The former European trade commissioner warned the UK would "pay an economic price" for leaving the EU and negotiations with former partners in Brussels would not be easy.
He asked: "Why should those who have just divorced us give us back the keys to the marital home as if nothing has changed?"
Lord Mandelson added the UK would be cut out of trade agreements struck between the European bloc and other nations, potentially meaning export tariffs of 20% or more on key UK products including "cars, machine goods, whisky and textiles" and the imposition of new import tariffs "raising the cost of imported goods and consumer prices for people in the shops".
The former commissioner added: "There is no rational or realistic way that Brexit offers a better set of global trade arrangements than those we have already."
"With the fate of future generations and our country's place in the world on the ballot paper, it is deeply irresponsible to pretend otherwise."
Matthew Elliott, chief executive of Vote Leave, said David Cameron had acknowledged that trade would continue if the UK broke away from the EU in the June 23 referendum.
He added: "Peter Mandelson should stop his scaremongering. It is safer to take back control and to start to spend our money on our priorities than it is to keep giving more power and money to the EU."