A leading Conservative is urging “ambitious” British firms to move to Malta after Brexit, triggering criticism that he is “selling our country down the river”.

Lord Ashcroft, a major party donor and former treasurer, hailed the Mediterranean island as a “superb location for UK companies needing an EU base”, once withdrawal is completed.

Theresa May has insisted she is “putting jobs at the heart of what we do in relation to Brexit”, arguing there will be a boost from the freedom to sign new trade deals.

But the peer admitted some companies would move “tens of thousands” of posts, saying: “Some UK firms have understandably decided, particularly in a period of uncertainty, that they will need a base in the EU in future.”

And he added: “Malta can match, or even better, other European countries in terms of what the island can offer to UK firms and its employees.”

The stance came under fire from Ian Murray, a Labour MP backing the People’s Vote campaign for a fresh referendum on the Brexit deal, who said: “Lord Ashcroft is selling our country down the river.

“He backs a Brexit that will devastate jobs and the economy at home while at the same time trying to flog the benefits of low-tax Malta to companies fleeing the mess he and his chums have made.”

And Gareth Thomas, a Labour supporter of the anti-Brexit Best for Britain group, said: “Brexiteer Lord Ashcroft is now recommending that British companies now move to Malta.

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“Malta, the EU’s smallest nation, which is routinely described a Europe’s ‘Pirate base for tax’. The front of Lord Ashcroft is astounding.”

Both MPs said the prospect of British companies moving some operations to stay in the EU made a further referendum “more important than ever”.

Lord Ashcroft has given many millions of pounds to the Conservative Party, including a £500,000 donation towards the party’s general election campaign last year.

He is also a former deputy chairman, who remains influential in British politics through his polling company and the website ConservativeHome, a blog aimed at grassroots party activists.

In an article on that website, he wrote: “As a hard-nosed, self-made businessman, I believe that Malta represents the best destination for ambitious UK firms that must have a post-Brexit presence in the European Union.

“I have long been – and firmly remain – pro-Brexit, and the UK itself will be the best location for nearly all UK companies.

“However, some UK firms have understandably decided, particularly in a period of uncertainty, that they will need a base in the EU in future.”

After touting Malta’s credentials, the peer added: “So is Malta the answer for all UK firms seeking to co-locate post-Brexit?

“In short: no. Giant companies who are looking to employ thousands or even tens of thousands of people in a EU base will almost certainly need to look elsewhere.”