Labour has been accused by the SNP of "siding with the Tories" in a row over whether civil servants should be working on the case for Scottish independence.
Ian Blackford, the Nationalist leader at Westminster, has written to Keir Starmer today to demand he distances himself from comments made by Ian Murray.
It emerged last week that 11 civil servants have been tasked with creating a new prospectus on Scottish independence by SNP ministers.
Murray - Labour's only MP in Scotland - yesterday wrote to Cabinet Secretary Simon Case, the ultimate head of the civil service in the UK, calling for an investigation that will “ultimately reverse” the decision.
But that move has prompted a furious reaction from the SNP, with the party claiming it has a clear mandate from voters to pursue plans for an IndyRef2.
Nicola Sturgeon has insisted she still plans to ask Scots the big constitutional question by the end of next year.
Blackford has now asked Starmer if he backs the position of Murray, who serves as Shadow Scottish secretary at Westminster.
"Last week you said you believed that Boris Johnson and his party were treating Scotland with 'utter disdain' and then went on to say a Westminster Labour government would give more powers to the Scottish Parliament," Blackford said in his letter to the UK Labour leader.
"Yet, your only MP in Scotland has written to the head of the UK civil service demanding that he should 'investigate and rule' on the Scottish Government for planning for an independence referendum – something for which the electorate of Scotland voted for at last May’s Holyrood election when they elected a majority of MSPs in support of a referendum – greater than that of 2011.
"This is an extraordinary move for any party which professes to respect democracy in Scotland, and one utterly at odds with the sentiments you expressed just last week."
He added: "In the interests of transparency, did you sign off on or in any way approve this letter? And if not, will you now publicly distance yourself from its contents?
"If you do not, then the only conclusion which people will draw is that – far from the claims of your rhetoric – you and your party stand shoulder to shoulder with Boris Johnson and are siding with the Tories in your joint efforts to thwart democracy and the will of the Scottish people."
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