There can be no one in Britain who is not outraged by the appalling attack on Sergei Skripal and his daughter Yulia in Salisbury last week. The use of military nerve agents on the streets of Britain is barbaric and beyond reckless. This horrific event demands first of all the most thorough and painstaking criminal investigation, conducted by our police and security services. They have a right to expect full support in their work, just as the public should also be able to expect calm heads and a measured response from their political leaders. 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 defies critics and calls for calm over Russia Read more

Theresa May was right on Monday to identify two possibilities for the source of the attack in Salisbury, given that the nerve agent used has been identified as of original Russian manufacture. Either this was a crime authored by the Russian state; or that state has allowed these deadly toxins to slip out of the control it has an obligation to exercise. If the latter, a connection to Russian mafia-like groups that have been allowed to gain a toehold in Britain cannot be excluded.

On Wednesday the prime minister ruled out neither option. Which of these ultimately prove to be the case is a matter for police and security professionals to determine. Hopefully the next step will be the arrest of those responsible.

As I said in parliament, the Russian authorities must be held to account on the basis of the evidence, and our response must be both decisive and proportionate. But let us not manufacture a division over Russia where none exists. Labour is of course no supporter of the Putin regime, its conservative authoritarianism, abuse of human rights or political and economic corruption. And we pay tribute to Russia’s many campaigners for social justice and human rights, including for LGBT rights.

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. Instead, Britain needs to uphold its laws and its values without reservation. And those should be allied to a foreign policy that uses every opportunity to reduce tensions and conflict wherever possible.

I will not step back from demanding that Russian money be excluded from our political system

This government’s diplomacy is failing the country. Unqualified support for Donald Trump and rolling out the red carpet for a Saudi despot not only betrays our values, it makes us less safe.

And our capacity to deal with outrages from Russia is compromised by the tidal wave of ill-gotten cash that Russian oligarchs – both allied with and opposed to the Russian government – have laundered through London over the past two decades. We must stop servicing Russian crony capitalism in Britain, and the corrupt billionaires who use London to protect their wealth.

So I will not step back from demanding that Russian money be excluded from our political system. We will be holding the government’s feet to the fire to fully back Labour’s proposed Magnitsky-style sanctions against human rights abusers, along with a wider crackdown on money laundering and tax avoidance.

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. We are willing to back further sanctions as and when the investigation into the Salisbury attack produces results.

But if we are to unite our allies behind action that needs taking, we must make full use of existing international treaties and procedures for dealing with chemical weapons. That means working through the Organisation for the Prohibition of Chemical Weapons to reduce the threat from these horrific weapons, including if necessary an investigation by chemical weapons inspectors into the distribution of Soviet-era weapons.

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 that continues to this day, while terrorism has spread across the globe.

The continuing fallout from the collapse of the Soviet Union and the virtual collapse of the Russian state in the 1990s must be addressed through international law and diplomacy if we are to reverse the drift to conflict.

Right now, the perpetrators of the Salisbury attack must be identified and held to account. Only through firm multilateral action can we ensure such a shocking crime never happens again.