Australia’s former top diplomat in the United Kingdom, Alexander Downer, has intervened in the British election contest, declaring the Morrison government would need to “substantially reduce” intelligence sharing with London in the event Jeremy Corbyn wins on 12 December.

In an excoriating assessment of the Labour leader at the National Press Club in Canberra, Downer contended that a Corbyn victory would imperil substantial Australian investments in Britain, and would trigger a reassessment of the “very intimate” security relationship between Canberra and London.

Downer is a former leader of the Liberal party in Australia, and was foreign affairs minister in the Howard government for more than a decade. He was appointed after his retirement from politics as high commissioner in London by the Liberal government in Canberra.

Downer on Tuesday rated Boris Johnson’s prospect of victory at about 70%, but he warned if the Conservatives failed to win outright “then expect Jeremy Corbyn to be the prime minister”. Downer said the result mattered to Australia because the UK was the second-largest offshore destination for Australian investment.

“If the Corbyn government comes in with its extremely radical economic agenda, a kind of Venezuelan-style, Maduro-style economic agenda, very South American economic agenda, and unfamiliar to any of us here in Australia – I think you would expect a very substantial deterioration in the British economy, and as a result of that, that would put at risk the investments we have … made in the UK,” Downer said.

I’m hoping ... that Boris Johnson wins the election on the 12th of December. I think it is important to us as Australians Alexander Downer

In relation to national security, he said the election of the leftwing Labour leader would disrupt the “very intimate and important relationship” between Asis, MI6, Asio, GCHQ and the Australian Signal Directorate. “Great Britain is the second-most important country in security terms in the western alliance”.

“Under a Corbyn government, they will abandon the support for the western alliance and steer a completely different foreign policy and security policy direction to such an extent that I think we would be unwise to continue the intelligence-sharing relationship with a Corbyn-led Britain of the kind that we’ve had … under Gordon Brown and Tony Blair and so on over the years.

“I think that would have to change. So I’m hoping, unusually … you will be surprised to hear me say this, that Boris Johnson wins the election on the 12th of December. I think it is important to us as Australians.”

Pressed to clarify his undiplomatic intervention on security sharing, Downer doubled down on Tuesday, saying: “I think we would substantially reduce it, substantially wind it back.

“We would have to be very careful because of the foreign policy stance of Jeremy Corbyn and the cohorts around him. These are people who are unsympathetic to and hostile to western interests, they have engaged as friends and allies of people like Hamas and Hezbollah, the Iranians, the Venezuelans and so on.

“These are people who are totally hostile to the traditions of western security policy. The leadership of the British Labour party is something we have never seen in a major political party in this country. These are people way over to the very, very far left, and it would change the whole nature of our engagement with the UK if Jeremy Corbyn became prime minister.

Comment has been sought from Australia’s foreign affairs minister, Marise Payne.

Downer’s appearance at Canberra’s National Press Club on Tuesday was billed as a situation report on Brexit, but the former diplomat and foreign affairs minister faced sustained questioning about his participation in the Barr inquiry in the United States.

Donald Trump has established the Barr inquiry to discredit the Mueller investigation. The Mueller investigation began after Downer was allegedly told by George Papadopoulos, then a Trump campaign aide, that Russia had obtained damaging information about Hillary Clinton from her emails.

Downer then informed US authorities, which has led to accusations from Trump allies, such as the Senate judiciary committee chairman, Lindsey Graham, about the then high commissioner’s “directed” role in relaying information to US authorities about the Clinton emails. Australia has rejected those accusations.

Downer refused to give direct answers to questions about the level of his participation in the Barr inquiry, or whether he now regretted his conversation in a London wine bar with Papadopoulos.

He said he did not want to “play into the toxic politics of America” or disrupt the orderly business of the inquiry by providing commentary, although he signalled without saying so directly that he had been interviewed, and was “fully cooperating with the Australian government”.

Downer declined to answer a question about whether he had sought permission from Canberra before he relayed the insights from Papadopoulos to the American chargé d’affaires in London in July 2016.

Guardian Australia revealed at the start of October that Downer had taken the then prime minister, Malcolm Turnbull, and senior colleagues by surprise when he shared politically sensitive information with his American counterpart in London.

Downer has not previously contested that version of events, and didn’t when asked if he wanted to set the record straight on Tuesday. “I’m not sure why you bother with it, I’m not sure why you care. I don’t want to set the record straight, I don’t care what people are saying,” he said.

“I retired from politics long ago, you can say what you like”.