Lord McFall of Alcluith, in a letter to the two candidates competing to replace Mr Johnson as prime minister, warns that any attempt to swell the ranks of peers “undermines public confidence in our parliamentary system”.
The letter, seen by the Sunday Times, comes amid reports that Mr Johnson is planning to add further members to the House of Lords as part of a political honours list before he departs Downing Street.
Those reports, which are referenced by Lord McFall in his letter to the two candidates, has already prompted concerns among opposition parties.
Baroness Hayman joined the warnings to the prime minister on Sunday, telling the BBC’s The World This Weekend programme: “The proposal is part of a trend to trash constitutional norms. So, in that sense, I think it’s a very bad idea.
“It has all sorts of grave consequences. And I’ve actually been surprised at how much anger there is across the House, basically, to put a large number of predominantly Tory peers into the House now.
“The House of Lords has provided difficulties for the Government. No Government likes that. Most prime ministers understand that it’s actually good for democracy for that to happen.
“I’m not sure Boris Johnson understands that having a challenging House of Lords actually improves Government policy and improves legislation.”
Lord McFall, who in the letter praises the “exercise of restraint” shown by former prime minister Theresa May in her approach to Lords appointments, is critical of Mr Johnson for not following suit.
He continues: “The current Prime Minister has taken a different approach to his predecessor, choosing not to exercise that same restraint, and making a significant number of appointments during his tenure – 86 in total, so far, which is far more than the committee recommended.
“As a result, despite the high number of retirements, there has been little progress in reducing the size of the House, which currently has over 800 members.”
Asking the pair to address the size of the Lords when one of them takes office, he tells them: “I am sure you agree that public trust in politics and in our Parliament and constitution is crucial going forward.”
Lord Norton of Louth, one of the foremost experts on parliament and the British constitution, told the same programme that the creation of new peers should be about “quality, not quantity”.
He has a Private Member’s Bill that would grant statutory powers to the House of Lords Appointments Commission, preventing future prime ministers from appointing new peers before the commission advises on their suitability.