Peter Mandelson
Lord Peter Mandelson claimed it would take many years to strike a trade deal with the EU. Reuters

Labour grandee Lord Peter Mandelson has attacked anti-EU activists for apparently trying to sell the UK electorate a "fantasy" of what a Brexit would look like.

The former European Commissioner for Trade warned it would take "many years" to strike a trade deal with the EU if the UK broke away from Brussels after the 23 June referendum. "Brexiters are trying to sell people a fantasy of what life outside of the EU would look like, without any evidence to back up their assertions," Mandelson told the Institute of Chartered Accountants in England and Wales.

"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."

Brexiters are trying to sell people a fantasy of what life outside of the EU would look like, without any evidence to back up their assertions.
- Lord Peter Mandelson

The former British business secretary also claimed that a post-Brexit Free Trade Agreement (FTA) with the EU would be worse than the single market access the UK enjoys today.

"Anyone arguing that special dispensation would be given to Britain needs to consider the irony of those who say we must leave the EU because we are so weakened within it yet claim that such fundamental change could be achieved from the outside when we leave," Mandelson declared.

He added: "You can be as ambitious as you like in trade policy but without leverage, without scale, you are not even at the table. And in commercial negotiations worth billions, if you are not at the table you are often on the menu."

Read more: The who, what, when and why of the historic Brexit vote

David Davis' vision

David Davis and David Cameron
David Davis and David Cameron Getty

The peer's speech comes as pro-EU campaigners increasingly press 'leave' backers over what a Brexit would look like.

Former Europe minister David Davis spelled out his own Brexit vision during a detailed briefing on 4 February.

The former Tory leadership contender claimed it was too risky for the UK to stay inside the EU and cited Switzerland as a positive case study for trading with the economic and political union.

"Switzerland is a small country surrounded by the EU. Its trade is absolutely dominated by the EU – more than 62% of its exports go to Europe. It runs a large trade surplus, and it is not big enough to be a critical market for any EU nation," Davis argued.

"The negotiation between the EU and Switzerland in the 1990s was marked by some hostility after it rejected EU membership, and yet it struck a decent deal.

"The optimum aim for us would be similar, but without the free movement of peoples. That would not be on the table. Essentially, we would be looking for a full-scale FTA."

But Brexit campaigners are yet to find consistency over the issue as Vote Leave and the Grassroots Out Movement battle it out for the Electoral Commission's official 'leave' campaign designation.

The latest online opinion poll from OBR, of more than 2,000 people between 24 and 25 February, put 'leave' four points ahead of 'remain' (52% versus 48%).