The claims that British trade with the Commonwealth can make up for leaving the EU is “the nuttiest of the many nutty arguments” advanced by Brexit supporters and “utter bollocks”, the former Australian prime minister Kevin Rudd has said.

In a lacerating piece for the Guardian, Rudd dismissed the claims by some Brexit supporters that the UK could strike deals with his country, New Zealand, Canada and India to soften the blow and said the UK risked undermining western values by leaving the EU in a weaker position when it left.

“I’m struck, as the British parliament moves towards the endgame on Brexit, with the number of times Australia, Canada, New Zealand and India have been advanced by the Brexiteers in the public debate as magical alternatives to Britain’s current trade and investment relationship with the European Union,” he wrote.

“This is the nuttiest of the many nutty arguments that have emerged from the Land of Hope and Glory set now masquerading as the authentic standard-bearers of British patriotism. It’s utter bollocks.”

Of the prospect of a free trade deal with Delhi, he writes: “As for India, good luck!”

Rudd, a former chair of the Commonwealth Ministerial Action Group, noted that, while Australia, Canada and New Zealand would remain supportive of a post-Brexit UK, their combined population of about 65 million people does not “come within a bull’s roar of Britain’s adjacent market of 450 million Europeans”.

And he cast serious doubt on suggestions the UK could quickly come to a free trade agreement (FTA) with India, pointing out that talks he began with the nation on behalf of Australia a decade ago are still going on. “A substantive India-UK FTA is the ultimate mirage constructed by the Brexiteers. It’s as credible as the ad they plastered on the side of that big red bus about the £350m Britain was allegedly paying to Brussels each week. Not.”

Rudd was writing as the British prime minister, Theresa May, made a last-ditch dash to Strasbourg in an effort to negotiate a form of Brexit she could get past MPs, who have overwhelmingly rejected her approach in the Commons. The government has indicated that it intends to honour its commitment to hold a meaningful vote on her Brexit deal on Tuesday, with less than three weeks remaining before the nation leaves the EU.

Rudd, who was Australian prime minister from 2007 to 2010 and again for a short spell in 2013, urged the UK to “use this critical fortnight to start turning all this around”.

He argued that there was a reason to do so “beyond Britain’s own demonstrable economic self-interest”, claiming that a weaker Europe – alongside a more inward-looking US – could only lead to a weakening of the “continuing idea of ‘the west’”.

“If the American pillar is looking a little shaky, Europe is looking even shakier. The strength of the political emergence of the far right across Europe’s four largest countries and economies – Germany, France, Italy and Spain – is frightening. The European political centre is being hollowed out,” he wrote.

“For Britain’s economic self-interest, as well as the wider political interests of the western community of nations, Britain should remain in the EU.”