A future Scottish independence referendum backed by Labour would contain a third option on federalism, it has emerged.
Shadow Scotland office minister Paul Sweeney insisted that it was not a "binary" issue as party continues to reel from the shock of Shadow Chancellor John McDonnell's revelation this week that it would not block a second vote on Scots leaving the UK.
But Labour's Edinburgh MSP Daniel Johnson insisted that the shift in position has created a crisis in the party.
Read more: Labour at war over split on Scottish independence vote
Mr Sweeney, the Glasgow North East MP, told BBC Radio Scotland a Labour government would deliver “a radical transformation and federate the United Kingdom”.
He said: “That of course should be a question in that referendum in future. It shouldn’t just be a binary referendum, and we’ll be backing that option.”
Senior Labour figures including former Prime Minister Gordon Brown, have backed a more federal approach to the united Kingdom, but such a move could depend on votes passing elsewhere in the UK.
Read more: John McDonnell's independence remark is causing Labour party crisis – Daniel Johnson
The call for a question on federalism to be included in any future independence referendum was backed by labour's Lothians MSP Neil Findlay.
He pledged to "vigorously oppose" independence in any future campaign.
But he said: "If there's a majority in the Scottish parliament and parties have stood on a manifesto saying that they will have a referendum and if those parties get a majority, then I think it's just democratic to say that they would then be allowed to negotiate with the UK Government to have that referendum."
However, McDonnell's intervention has come under fire from the Edinburgh Southern MSP Daniel Johnson who accused the Shadow Chancellor of "running rough-shod over the leadership and policymaking bodies of the party in Scotland."
He added: "Richard Leonard has been very clear and very strong on the constitutional question. A new referendum is “not needed and not wanted.
"Party conference has passed policy. Yet it would appear McDonnell only discussed this with the leader of the Scottish party after he had made his views public.
"McDonnell is not only proposing more constitutional crisis as an answer to the current one – he is creating a crisis within the Labour Party itself."