Jeremy Corbyn's spokesman has said there is not yet definitive proof the Russian state was behind the attempted murder of a former spy in Salisbury.
He said the most likely explanation was Russia that was "directly or indirectly responsible" for the attack but "culpability takes many forms".
He pointed to the "problematic history" of UK intelligence on chemical weapons.
It came as several Labour MPs called for their leader to be firmer in his condemnation of Russia.
A succession of Labour MPs – including many who are on record opposing Mr Corbyn as Labour leader – backed Theresa May's decision to expel 23 Russian diplomats and take other measures in response to the poisoning of Sergei Skripal and his daughter Yulia with a military grade nerve agent.
Russia has denied any involvement in the attack.
In his response to the prime minister's Commons statement announcing the measures, Mr Corbyn called for a "decisive and proportionate response based on clear evidence".
His former leadership contest rival – and current chairwoman of the Home Affairs committee – Yvette Cooper said what was needed was "unequivocal" condemnation of Russia's actions.
And former Labour minister Pat McFadden said showing resolve when your country was threatened was an "essential component of political leadership" and "more than words" was needed as a response.
Mr Corbyn's spokesman said Labour did not oppose any of the measures outlined by Mrs May – including the expulsions – but urged caution over reaching a definitive judgement on who was behind the attack.
"Whoever carried out the attack is responsible for a heinous and reckless attack in a civilian area," he told reporters after the Commons debate.
He said the Labour leadership could not disclose the detail of the security briefings it had received on the crisis.
But he said the two scenarios put forward by the prime minister on Monday – either direct Russian state involvement or a loss of control by the state of the deadly chemicals – were still both possible and that "culpability had different forms".
He said there was "a series of possibilities as to who may be responsible", including oligarchs, Mafia elements and other ex-Soviet states.
In a reference to intelligence failings before the Iraq War, he added: "There is a history between weapons of mass destruction and intelligence which is problematic, to put it mildly,"
Asked if Mr Corbyn, who opposed the then Labour government's backing for the Iraq War, was worried about being out of step with his party, the spokesman said Mr Corbyn had been "right about a lot of things" in the past two decades and the reaction of other MPs "isn't always backed by the facts".
Speaking in the Commons, Mr Corbyn had condemned the "appalling" attack but did not directly apportion blame to Russia.
He pressed Theresa May on whether the UK had supplied traces of the nerve agent used in the attack to Russia for analysis before Wednesday's deadline, as the Kremlin had asked.
And he asked what action the UK was taking with its allies through the Organisation for the Prevention of Chemical Weapons.
The UK's response, said the Labour leader, should be underpinned by support for the rule of law and international agreements and respect for human rights.
But Labour MP John Woodcock – a long-term critic of Mr Corbyn – said it was vital for the Commons to "speak as one" on the issue and show its resolve in the face of Russian aggression.
He told the PM she will be "reassured to hear that a clear majority of Labour MPs, alongside the leader of every other party, support, the firm stance she is taking".
His colleague Ben Bradshaw said "most of us on these benches fully support the measures she has announced and, indeed, some of us think they could have come a bit sooner."
Sixteen Labour MPs have signed a Commons motion "unequivocally accepting" the Russian state's culpability for the attack and supporting the UK government's response.
One of the signatories, Chuka Umunna, said MPs must make clear where they stood.
Skip Twitter post by @ChukaUmunna
Have read the comments of the Leader of the Opposition’s spokesperson. Mr Milne’s comments do not represent the views of the majority of our voters, members or MPs. We’ll get abuse for saying so but where British lives have been put at risk it is important to be clear about this.
— Chuka Umunna (@ChukaUmunna) March 14, 2018
End of Twitter post by @ChukaUmunna
But Mr Corbyn has been defended by former Labour foreign secretary Dame Margaret Beckett, who said he was right to press the PM on the details of her communications with Moscow over the past 24 hours.
"To ask how she handled that is a legitimate question," she told BBC Radio Derby. "He is trying, I think, to get a fuller picture as to what the contacts are."