A group of Labour backbenchers has said it "unequivocally accepts" the Russian state's culpability for the Salisbury attack after Jeremy Corbyn's team raised doubts about who was responsible.
The Labour leader's spokesman said there was a "problematic" history over the use of UK intelligence and left open the possibility of the Federation being framed.
Speaking to reporters after the statement updating MPs on the poisoning in Salisbury, the spokesman indicated that Labour does not believe there is enough evidence yet to blame the Russian state for the attack.
Theresa May condemned the comments as "shocking" and "outrageous" after a Conservative MP raised them in the Commons chamber.
The spokesman's comments prompted Labour backbencher John Woodcock to table an Early Day Motion "unequivocally" accepting the "Russian state's culpability" for the attack, and supporting "fully" the statement made by Mrs May in the Commons.
The motion was swiftly signed by a number of prominent critics of Mr Corbyn, some of whom went public with their criticism of the leader's senior aide Seumas Milne.
Labour MP Anna Turley tweeted: "I'm afraid Seumas doesn't speak for my Labour or British values", while Chuka Umunna said: "Mr Milne's comments do not represent the views of the majority of our voters, members or MPs".
The Labour leader has been given security briefings on the incident, but the spokesman said the Government may have more information.
He told reporters: "The Government has access to information and intelligence on this matter which others don't.
"However, also there is a history in relation to weapons of mass destruction and intelligence which is problematic, to put it mildly.
"So, I think the right approach is to seek the evidence to follow international treaties, particularly in relation to prohibitive chemical weapons."
Asked if he could rule out the possibility of Russia being framed, the spokesman said the evidence pointed "overwhelmingly" to the two options set out by the PM.
"In the meantime I think it is essential we follow the evidence and what the evidence produces," he added.
Pressed on whether another former Soviet state, such as Ukraine, may be the origin of the substance, he replied: "I think the second option Theresa May set out on Monday and again today, that the Russian government had lost control of weapons grade nerve agents which may have been produced during the Soviet period, contains within it a series of different possibilities of who then might have been directly responsible for that.
"If the material is from the Soviet period, the break-up of the Soviet state led to all sorts of military material ending up in random hands."
The spokesman said that during the "WMD saga" there was "both what was actually produced by the intelligence services, which in the end we had access to, and then there was how that was used in the public domain in politics.
"So, there is a history of problems in relation to interpreting that evidence but, in this case, the Government may well have other evidence that we are not aware of. Clearly this issue has to be followed on the basis of the evidence."
The PM criticised the Opposition leader's spokesman from the despatch box.
She said: "I am surprised and shocked at the statement that has been put out by the spokesman for the leader of the Opposition, and as I was going to say it is very clear I think from the remarks that have been made by backbenchers from the Labour Party that they will be equally concerned about that remark.
"They stand full square behind the Government in the analysis that we have shown and the action that we have taken."
She added: "What we are talking about here in the United Kingdom is the use of a chemical weapon, a nerve agent, a military grade nerve agent against people here in the United Kingdom.
"That is very clear and I think it is quite wrong and outrageous that the leader of the opposition's spokesman made the comments in the relation to this that he has."
Mr Woodcock said: "This is a time for the nation to speak as one so it was heartening that the overwhelming majority of MPs of all parties spoke so strongly in support of the strong package of measures outlined by the Prime Minister today.
"There should be no doubt over Russia's culpability and we should resist the Russian propaganda machine's attempts to sow the seeds of doubt in the minds of the British people. So we hope as many MPs as possible sign our early day motion making clear we hold the Russian state squarely to blame for this atrocity and support the Government's response.
"Support for strong action from members of parliament of all sides is necessary to face down this threat to our nation and the international rules-based system that Putin seeks to destroy."