Jeremy Corbyn has defied critics in his own party and warned the prime minister against “rushing way ahead of the evidence” over the Salisbury poisoning, in what he called the “fevered” atmosphere of Westminster.

The Labour leader used an article in the Guardian to urge the government to take a “calm, measured” approach – and warn against the drift towards a “new cold war” with Russia.

Corbyn was criticised by some in his own party on Wednesday for what they regarded as his lacklustre response to Theresa May’s announcement of action against Moscow over the poisoning of former spy Sergei Skripal and his daughter, Yulia, with a nerve agent.

The Labour leader doubled down on that cautious approach on Thursday night. “This horrific event demands first of all the most thorough and painstaking criminal investigation, conducted by our police and security services,” he said.

“To rush way ahead of the evidence being gathered by the police, in a fevered parliamentary atmosphere, serves neither justice nor our national security.”

Corbyn warned against a “McCarthyite intolerance of dissent” over Russia. “Labour is of course no supporter of the Putin regime, its conservative authoritarianism, abuse of human rights or political and economic corruption,” he said.

“However, that does not mean we should resign ourselves to a ‘new cold war’ of escalating arms spending, proxy conflicts across the globe and a McCarthyite intolerance of dissent.”

'They have just one week to leave': May expels 23 Russian diplomats – video

Corbyn backed May’s decision to expel 23 diplomats, but suggested a financial crackdown on Russian oligarchs would be more effective.

“We agree with the government’s action in relation to Russian diplomats, but measures to tackle the oligarchs and their loot would have a far greater impact on Russia’s elite than limited tit-for-tat expulsions,” he said.

The shadow chancellor, John McDonnell, also pledged to “hammer” money-laundering, saying that would be a more effective weapon against Vladimir Putin’s regime than expelling diplomats.

He claimed Labour would be tougher on the Putin regime than the government, by starving “gangster politicians” of cash, outlining anti money-laundering measures designed to stop the channelling of stolen money through the City and tax havens under UK jurisdiction.

However, despite McDonnell’s tough stance, Corbyn’s remarks are likely to reignite tensions in the parliamentary Labour party over the leadership’s tough response to the Russia crisis.

Nineteen Labour MPs have signed an early day motion, tabled by Barrow MP John Woodcock – a longtime Corbyn critic – putting on the record their full backing for May and the measures she announced.

Some shadow cabinet members appeared to take a different stance, with Emily Thornberry insisting Russia had a “prima facie case” to answer, to which it had offered no defence.

The shadow defence secretary, Nia Griffith, publicly questioned the connection drawn by Corbyn’s spokesman, between intelligence in the Salisbury case and the “dodgy dossier” that helped make the case for the Iraq war. “I think the two situations are very different,” she told the BBC’s Today programme.

But the Corbyn’s article made that link explicitly: “There can and should be the basis for a common political response to this crime. But in my years in parliament, I have seen clear thinking in an international crisis overwhelmed by emotion and hasty judgments too many times,.”

“Flawed intelligence and dodgy dossiers led to the calamity of the Iraq invasion. There was overwhelming bipartisan support for attacking Libya, but it proved to be wrong. A universal repugnance at the 9/11 attacks led to a war on Afghanistan, which continues to this day, while terrorism has spread across the globe.”

The Guardian understands neither Griffith nor Thornberry were shown the article in advance. They had been reassured by an interview Corbyn gave earlier on Thursday, in which he said that “evidence points towards Russia”.

Corbyn’s allies have leapt to his defence, with the Brighton Kemptown MP Lloyd Russell-Moyle tweeting that the Labour leader’s detractors were “rightwing sods” who want to “beat the drums of war without a clear plan”.

Woodcock said: “There is much in this piece which is troubling, not least the strange reference to McCarthyism. Jeremy Corbyn is not a powerless victim, he is a man who seeks to lead our country at a time when it has been shown to be under significant threat.”

McDonnell said he had been urging tougher action against money laundering in the City and Britain’s overseas territories but had been repeatedly rebuffed by the government. Six Labour amendments to the sanctions and money laundering bill had been opposed by the Tories in the past two weeks, he added.

“I will be tougher on Russian money than the government and clearer about how to do it,” McDonnell said.

The shadow chancellor said that would involve tightening the definition of a “politically exposed person” – someone in a public position viewed as being at higher risk of bribery and corruption – to include a wider range of people close to Putin.