A Canadian-style trade deal with the EU would take years to achieve and leave Britain paying "more for less", Lord Mandelson is to warn.
The former business secretary will say those who want to use the agreement as a post-Brexit model are condemning the UK to prolonged uncertainty, higher trade tariffs and lost influence.
In a speech to the British Irish Chamber of Commerce, the Labour peer will highlight comments by London Mayor Boris Johnson during a visit to a trucking firm in Kent on Friday.
"He finally revealed what Brexit looks like - and I don't mean the sight of his rear climbing in to that driver's cab," Lord Mandelson will say.
"The Leave campaign's favoured alternative model is Canada's trade agreement with the EU.
"This is a major strategic error by the leave campaigns, as it exposes the inferior, weaker alternative future they offer."
Lord Mandelson will tell the audience in London that he started "scoping" a trade deal with Canada nine years ago when he was a European Commissioner, and the negotiations have only just come to a conclusion.
"This agreement offers nothing like unhindered access to the Single Market," he will say. "Farmers face restrictions, carmakers face cumbersome rules.
"Some EU tariffs are phased out only after seven years. There are 700 pages of exemptions from free trade, with vital sectors such as audio visual and air transport excluded.
"And as Canada is outside the Single Market it has no input to EU regulations, meaning its products, for example medical equipment, must be approved by EU authorities before they can enter European markets.
"Most significant of all, financial services are not 'passported' into the EU. If we had similar terms, this would spell disaster for Britain's financial sector.
"That is the Canada deal: new tariffs on trade, no influence and restricted market access. In short, you pay more for less."
Lord Mandelson will add: "What Boris doesn't understand - or wilfully ignores - is that trade deals are elaborate compromises from which those with the greatest negotiating power benefit most and countries even of our size and proximity ultimately have to settle for what they're offered.
"That's the real world rather than the fantasy world Boris and his fellow Brexiters occupy.
"If we opted for Boris's 'free trade' model, it would take a couple of years to scope out the sort of agreement Britain could get. Then the details and the transitional arrangements have to be argued over - in Canada's case this took seven years to hammer out.
"Finally, the agreement has to be ratified by the European Parliament (good luck), and potentially all 28 national parliaments (hardly a cake walk).
"So I say to Boris and his friends: think carefully and look before you leap."