Keith Brown MSP has written to Cabinet Secretary Sir Mark Sedwill asking him to investigate how a letter he had sent to Mr Walker had ended up in the office of Scottish Conservative MP Luke Graham.

The confidential correspondence had raised a constituent’s concerns about the impact of Brexit on their business which relied on EU workers. Mr Brown became aware that his letter had been leaked, after receiving a call from Mr Graham’s office asking for more details.

An inquiry has already been conducted by the Department for Exiting the EU’s permanent secretary Clare Moriarty, after an initial complaint by Mr Brown, who found there had been “unauthorised disclosure” of information “by a member of staff”.

SNP depute leader Keith Brown. Picture: John Devlin

In her letter Ms Moriarty also said that “appropriate action” had since been taken, but a spokesperson for DExEU refused to detail what that involved.

Meanwhile, Mr Graham, whose Westminster constituency of Ochil and South Perthshire overlaps with Mr Brown’s Holyrood constituency of Clackmannanshire and Dunblane, said he believed it was “standard practice” to include the local MP into correspondence which referenced their constituency.

The Scotsman understands that Keith Brown has now escalated his complaint and written to Sir Mark Sedwill amid concerns the breaching of confidentiality could be “widespread”.

Mr Brown said: “This is a deeply concerning incident which should never have occurred. Constituents raise concerns with their MSPs in complete confidence. It is unacceptable for strict data protection rules to be cast aside for political gain – which appears to have been the motivation here.”

He added: “While I welcome the findings of the Permanent Secretary to the Brexit department that an unauthorised disclosure did indeed take place, the case raises more questions than it answers. Just how widespread are these practices? Are Tory ministers regularly divulging confidential information to Tory MPs for political expediency?

“It demonstrates that the whole Westminster system is completely broken – and that goes right to the heart of the department that is supposed to be handling Brexit.”

A spokesperson for DExEU said that the department had “rigorous processes in place to address issues of this sort, and this was dealt with immediately when it came to light.”

Mr Graham said he had been “informed” about a constituent issue but he had “received no personal constituent data”. He added: “My office called Keith Brown’s to confirm details and ask if his office would give further details about the case so we could support. My office has a strong record of working with all levels of government to resolve constituent issues and I will continue to do so.”

SNP demand inquiry into ‘data breach’ of letter to Brexit minister