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.
There have been reports Mr Bercow – who previously sat as a Conservative MP – has been nominated for a peerage by the outgoing Labour leader Jeremy Corbyn.
Lord Lisvane began working at the House of Commons in 1972 and has held a number of procedural roles, including clerk for Private Members’ Bills and clerk for the European Scrutiny Committee.
He became Clerk Assistant and Director General of the Chamber and Committee Services in 2009, before stepping up to the Clerk of the House of Commons in 2011.
After standing down in 2014, he became a life peer and sits in the House of Lords as a crossbencher.
‘Taken into account’
Former Leader of the Commons, Andrea Leadsom, said anyone found to have bullied or harassed colleagues in Parliament “should not be offered a peerage”.
The now-business secretary, who clashed with Mr Bercow on a number of occasions in the Commons, told BBC Radio 5 Live: “I worked cross-party to put in place a complaints procedure which would mean anybody, whoever they are, including the Speaker of the House of Commons, or indeed of the Lords, would be subject to those same complaints procedures.
“Specifically, we made sure if you were an MP and you were potentially going to be offered a peerage, that anything that was alleged against you would be taken into account.
“So, I think it is really important, whoever you are, that all of those complaints are taken seriously.”