Boris Johnson’s plan to hand out large numbers of peerages before stepping down as prime minister has been denounced by a former Speaker of the House of Lords as “part of a trend to trash constitutional norms”.

Baroness Hayman said there was anger across the upper house at the prospect of dozens of the PM’s friends and allies being made lords.

And another ex-Lord Speaker, Lord Fowler – formerly a Conservative cabinet minister – said the appointment of Tory donors was bringing the system into “some kind of contempt” and undermining public support for the unelected chamber.

Reports suggest that the outgoing prime minister is planning to create dozens of new lords in a political honours list in the coming weeks, followed by a resignation honours list when he finally steps down in September.

His plans have prompted the current Lord Speaker to write to the contenders to succeed Johnson to appeal for “restraint” in the creation of new peers in a house that has more than 800 members at a time when the stated ambition is to reduce it to 600.

Lord McFall urged Liz Truss and Rishi Sunak to follow the example of Theresa May, who appointed just 43 peers in her three years in office, rather than Johnson, who has already ennobled 86.

Among those said to feature in Mr Johnson’s latest plans for peerages are billionaire Tory donor Michael Hintze, former Daily Mail editor Paul Dacre and Winston Churchill’s grandson Sir Nicholas Soames.