Unite members on strike march through the streets of Glasgow

to the McCluskey letter. In a written response, one branch secretary has demanded to know how Unite got the addresses of Labour members who are not in the union: “Many members are concerned you have their contact details.

“It appears you may be using a Labour Party membership list. It may be that you have conformed in some way to the requirements of the Data Protection Act, but I hope you will understand my deep concern.”

Unite member and councillor Dennis Goldie has also written to McCluskey: “Labour Party members who are not Unite members are receiving letters from you, leaving them to wonder if their data is safe.”

An SNP spokesperson said: “It suggests a real lack of faith if Labour’s Scottish Executive has been bypassed by party bosses in Westminster. Johann Lamont claims she is leader of the Labour Party in Scotland, but this would suggest her leadership is tenuous.

“With so many murky dealings, you would think Labour would try to encourage transparency instead of hushing up their report.”

A Scottish Labour spokesman declined to comment.

A Unite spokesman said: “We have sent letters to all Unite members of the CLP and are investigating the possibility that non-members may have received a letter in error, although we have not received any direct complaints.”