Liberal Democrat MP Christine Jardine said the Labour Party risks ostracising their Scottish supporters by allowing a second independence referendum in the country.
Ms Jardine, the MP for Edinburgh West, said a pledge by Labour's shadow chancellor to allow Holyrood a second vote on their membership of the UK if the party were to win power at a general election was "nonsense".
"He [John McDonnell] is out of touch with what is happening, certainly in Scotland," Ms Jardine told talkRADIO's Julia Hartley-Brewer.
"If the Labour Party were to go into an alliance with the SNP, and if part of that deal was that the SNP would get the second independence referendum, there's a large section of the Scottish public who would find it very difficult to forgive the Labour Party for that."
She continued: "The majority of people in Scotland want to remain in the UK and want to remain in Europe, which is a Liberal Democrat position."
John McDonnell's pledge was not welcomed by the leader of Scottish Labour, Richard Leonard, who is opposed to another vote.
"I was elected 18 months ago directly by members of the Scottish Labour Party, and one of the parts of the platform I stood on was clear opposition to a second independence referendum," he said.
And Labour MP Jim Fitzpatrick accused Mr McDonnell for "making policy on the hoof" to appease the SNP.
He told talkRADIO on Wednesday: "When we had the election for Scottish Independence in 2014, it was said at the time it would be a once-in-a-generational referendum.
"John has clearly thrown that out of the window off his own back."