The SNP depute leader has urged the head of the Civil Service to investigate after a UK Government department admitted a data breach by passing information from the SNP to a Tory MP.

The Department for Exiting the EU (DExEU) said there was “unauthorised disclosure of information by a member of staff” after Keith Brown wrote to Brexit Minister Robin Walker raising the concerns of a constituent.

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Mr Brown has now written to Sir Mark Sedwill saying the breach shows the Westminster system is “completely broken” and asking him to investigate how widespread the practice is.

The row was sparked after Clackmannanshire and Dunblane MSP Mr Brown wrote to Mr Walker raising the concerns of a constituent whose business relies on EU workers.

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The MSP’s office was then called by a member of staff for Luke Graham MP, whose Ochil and South Perthshire constituency overlaps with Mr Brown’s, asking for more information about the constituent.

Mr Brown complained to Mr Walker and an investigation was carried out.

In a letter, DExEU Permanent Secretary Claire Moriarty told the MSP: “We have now completed that investigation and I can confirm that there was an unauthorised disclosure of information by a member of staff.”

She said Mr Graham had been asked to delete the communication from his records.

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.

“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?

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“And why are Tory MPs getting priority over SNP members raising legitimate concerns on behalf of their constituents? Are UK ministers operating a dual speed system which treats MSPs as second class?

“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 DExEU spokesman said: “The department has rigorous processes in place to address issues of this sort, and this was dealt with immediately when it came to light.

“Appropriate action has been taken in line with our security, data protection and disciplinary procedures.”

Luke Graham MPLuke Graham said no personal constituent data was disclosed (Chris McAndrew/UK Parliament/PA)

Mr Graham told The Herald he was “informed there was a constituent issue that may have not been responded to, but received no personal constituent data”.

He added: “My office called Keith Brown’s office to confirm details about the issue and ask if his office would give further details about the case so we could support.

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“My office has a strong record of working with all levels of Government to resolve constituent issues/concerns…

“It is also standard practice to include the local MP in any correspondence referencing that MP’s office. I received no such correspondence from Mr Brown.”