Shadow defence secretary Nia Griffith has said she “can’t speak for Seumas Milne”, adding that Jeremy Corbyn’s spokesman “has to speak for himself”.
He said on Wednesday there was not yet definitive proof the Russian state carried out the spy poisoning.
It followed a number of Labour MPs being at odds with the Labour leader’s response to Theresa May’s statement on the Russian spy case.
Ms Griffith told the BBC the party accepted “Russia was responsible”.
Moscow denies responsibility and says it is working on retaliatory measures.
Speaking on BBC Radio 4’s Today programme, Ms Griffith said: “We very much accept that what the prime minister said – this is a very sophisticated nerve agent and Russia is responsible for this attack and therefore we’re fully supporting the measures which the government is taking, including the expulsion of 23 diplomats.”
Asked if she was at odds with Mr Corbyn on the issue, Ms Griffith said she was more “plain speaking” than her leader and said he had made it clear in “the subsequent statement that he put out” on Wednesday evening that Labour was “fully supportive” of the Government’s actions.
“I think the important thing is that is our position now,” she added.
Mr Corbyn’s spokesman’s comments came in a briefing to reporters after the Commons statement in which Theresa May set out a series of measures – including the expulsion of the Russian diplomats – in response to the poisoning of Sergei Skripal and his daughter Yulia in Salisbury last week.
He 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 that Russia was “directly or indirectly responsible” for the attack but “culpability takes many forms”.
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,”
Corbyn Facebook statement
Speaking in the Commons, Mr Corbyn condemned the “appalling” attack but 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 in a later Facebook post, Mr Corbyn called for the Russian authorities to “be held to account on the basis of the evidence and our response must be both decisive and proportionate”.
Ms Griffith said that statement made it “very clear that we’re fully supportive of the government’s actions because clearly we accept both what the prime minister has said and that Russia is responsible”.
Asked why the Labour leader had not given his unequivocal support of the government in the Commons, she said: “He’s made it clear in subsequent statements. The important thing is that this is our position now and I can assure you very strongly that is our frontbench position.”