A complaint has been made against former Commons Speaker John Bercow by a previous senior advisor.
Lord Lisvane, who served as Clerk of the House between 2011 and 2014, said he submitted the formal complaint to the Parliamentary Commissioner for Standards on Wednesday.
It is believed to centre on bullying.
Mr Bercow, who stood down from his role last year, said Lord Lisvane had “ample opportunity” to raise issues in their five years working together.
In a statement, Mr Bercow said: “At no stage did he do so, even though he became Clerk of the House – the most senior official. The timing of this intervention is curious.”
The former Speaker, who spent 10 years in the role, has faced other accusations of bullying during his time in office, but has denied all the claims.