Ruth Davidson has said that she undertook “due diligence” before accepting the new role

Ruth Davidson must resign as an MSP if she proceeds with her decision to join a leading PR company, a chairman of the industry body has warned.

George McGregor, co-chairman of the UK Public Affairs Board, questioned the advice that the former Scottish Conservative leader received from Scottish parliamentary officials before taking a role with the lobbying company Tulchan.

Ms Davidson will earn £50,000 a year for 25 days’ work as a senior adviser. She said at the weekend that she had undertaken “due diligence” before accepting the new role, including meeting parliamentary staff at Holyrood.

Mr McGregor, who was a special adviser to Helen Liddell, now Baroness Liddell of Coatdyke, when she was Scottish secretary in Tony Blair’s government, described Tulchan as “outliers in