Eurosceptic Cabinet ministers complaining about their access to Government papers are lucky not to be sacked and should stop "whinging", former business secretary Lord Mandelson has said.
The former European trade commissioner also hit out at the "thoroughly reckless" approach of London Mayor Boris Johnson, warning that a British exit from the EU would damage the economy.
Labour peer Lord Mandelson said Brexit would lead to rising prices in British shops and higher barriers for exporters as the UK would lose access not just to Europe's single market but also the international trade deals signed by the EU.
At an event in the City of London he dismissed the concerns of pro-Brexit ministers over restrictions on access to official documents about the referendum.
In barbed comments about the row over the contents of "Chris Grayling's in-tray", he said: "We are faced in this country with the biggest choice of our generation, a choice which is going to have a huge impact on our jobs, our trade, our investment, our place in the world.
"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 Mr Johnson for criticism following the Mayor's comparison of himself to James Bond, standing up for Britain against the forces of Brussels.
The former Brussels official 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."
Lord Mandelson warned that the UK would "pay an economic price" for leaving the EU.
Leaders of EU countries and MEPs in the parliament would not make it easy to strike a trade deal with Brussels after a Brexit, he said.
"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 that as well as attempting to negotiate a deal with Brussels, which could require signing up to EU rules and regulations, the UK would be cut out of trade agreements struck between the European bloc and other nations.
That could mean new tariffs of 20% or more on key UK exports including "cars, machine goods, whisky and textiles".
He warned that under World Trade Organisation rules the UK could also be forced to impose import tariffs on those countries because it would no longer be covered by preferential EU deals.
"So one of the first acts of the UK outside the EU would actually be to increase border protections because we are no longer covered by EU agreements, raising the cost of imported goods and consumer prices for people in the shops," he said.
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."
The top civil servant in the department of pro-Brexit Northern Ireland Secretary Theresa Villiers told MPs that, under the terms of the guidance issued by Cabinet Secretary Sir Jeremy Heywood, the Secretary of State might have to use a freedom of information request or a parliamentary question to obtain information produced by her own staff.
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, Northern Ireland Office Permanent Secretary Sir Jonathan Stephens 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 House of Commons Public Administration and Constitutional Affairs Committee.
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".
While accepting that "some challenging issues" may arise, he insisted: "It's unusual, but I don't find it puts me in an impossible position, by any means."
The Civil Service was facing "a wholly exceptional situation" in which the Prime Minister had made clear he would allow ministers to oppose the position of the Government and campaign on a personal position, he said, adding: "Personally, I would struggle with an arrangement that says the Civil Service should support one particular personal position."
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".
"There is no sense in which this is getting in the way of the normal business of Government," said Sir Jonathan. "I'm confident that with common sense and goodwill on all sides, these arrangements can be made to work."
Committee chairman Bernard Jenkin told the mandarin: "I think if common sense were to prevail ... you would be saying to us very clearly that the normal functioning of the department prevails over any other consideration, because that's what the Secretary of State is there for, and if there was any doubt that the Secretary of State should see some information for the normal functioning of government, she should see it."
And Conservative committee member David Jones asked him: "It is the case, is it not, that as a consequence of the guidance, your duty to the Government as a whole will supersede your personal duty as a departmental civil servant to support your Secretary of State?"
Sir Jonathan replied: "My duty as a civil servant is to support my departmental minister in support of Government policy."