The leader of the Vote Leave anti-European Union campaign is talking "absolute rubbish" about Brexit, Lord Mandelson has said.
On Tuesday morning, the former Labour cabinet minister and EU trade commissioner said Lord Lawson was wrong to claim Britain would keep its current international trade deals if it voted to leave.
Rather than inheriting the agreements negotiated with other countries while part of the EU, the UK would have to start from scratch, Lord Mandelson argued.
"Nigel Lawson, not for the first time, is wrong. He is engaging, I'm afraid, in fantasy politics and we need more facts and less fantasy," he told BBC Radio 4's Today programme.
Lord Mandelson also warned it free trade deals between governments take "between four and seven years" or "sometimes much longer" to negotiate.
Pro-Brexit campaigners have argued trade deals, including one between the UK and the EU, could be completed much faster.
But Lord Mandelson told Today: "Any idiot can start a trade agreement. The question is where does it end up? You can deal with tariffs relatively easily. What is much harder to deal with are the behind the border regulatory barriers."
Conservative cabinet minister Chris Grayling, one of the leading campaigners for an 'Out' vote, yesterday dismissed claims Brexit would lead to ten years of uncertainty.
"Claims that it will take twice as long to sort out a free trade deal with the EU as it did to win world war two are clearly ludicrous," he said.
In a speech today, Lord Mandelson will warn British firms will face years of uncertainty and the possibility of tariff barriers on exports if the country votes to sever ties with Brussels.
The ex-business secretary will say that negotiating a free trade deal with the EU following a vote to Leave would be "harder than Brexiters think" because for every sympathetic politician in the other 27 member states there would be others who believe the UK "must not be given a quick or easy ride".
The Labour peer will warn that without the benefit of EU trade deals with international markets, British cars, whisky and textiles could attract tariffs of 20% or more.
"Being part of a powerful trade bloc is an advantage not a straitjacket and one with a long track record of success," he will insist. "The EU is probably the world's leading rule setter in international trade and this is a significant strength for Britain."
In a speech in the City of London, the former EU trade commissioner will say: "Brexiters cannot argue that we are weakened in the EU as it is but would suddenly be strong enough to dictate terms if we left. For every politician who saw the pragmatic case for dealing with the UK, there would be another who had little doubt that the UK must not be given a quick or easy ride."
Brussels, he believes, would insist on the UK signing up again to common standards in order to gain access to the single market following a vote to leave in the June 23 referendum.
"As a result, we would have left the EU in order to assert our national sovereignty only to find that, as a condition of access, we did not have independence from EU regulation after all," he will say.
Responding to Lord Mandelson’s comments today, Matthew Elliott, Chief Executive of Vote Leave said: "Peter Mandelson told us the British economy would fall off a cliff if we didn’t join the euro and now he is indulging in the same scaremongering about the referendum. He was wrong then and he is wrong now. He is starting to resemble a man wearing a sign saying the ‘end is nigh’.
"As the Prime Minister has said - trade will continue after we Vote Leave, 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."