David Cameron has created so many new peers since becoming prime minister that the effectiveness of the House of Lords has been damaged, a cross-party group of senior parliamentarians warned today.
A report backed by senior peers from all the major parties said there would be "disastrous consequences" unless immediate action was taken to reduce the size of the Lords.
It added that the coalition's aim of rebalancing the chamber so that it is proportional to vote share in last year's general election would be "foolish and unsustainable" at present.
The Prime Minister has created 117 new peers since last May, an increase unprecedented in recent times which has taken active membership of the Lords to 792.
Introducing "proportionality" - an objective set out in the Coalition Agreement - would mean taking the chamber to 1,062 members.
The report stated that the present course was "clearly unsustainable".
As well as the the rising cost associated with having more peers, the increase in members over the past year had had "significant effects on the chamber's functioning".
These included "overcrowded conditions", in terms of office space and even in the chamber itself, and a "more fractious atmosphere" as peers have to compete to take part in debates.
A move away from the traditionally "non-partisan ethos" had also been noted, it said.
"The House of Lords' effective functioning has already been compromised by the recent rapid rise in membership, in ways which are damaging its effectiveness," it said.
"Any further increase in size could fundamentally undermine the chamber's ability to do its job. Nor is there any desire in the country for an ever-larger parliament."
It added: "In order to avoid disastrous consequences for the House of Lords, and thus for parliament as a whole, we conclude that the current system of appointments must change, with immediate effect."
The report, House Full, was published today by University College London's Constitution Unit with the endorsement of peers including former Cabinet Secretary Lord Butler, former Commons Speaker Baroness Boothroyd, convenor of the independent crossbenchers Baroness De Souza, former Labour cabinet minister Lord Adonis, former Tory Cabinet minister Lord Forsyth, former Liberal leader Lord Steel of Aikwood, and former Master of the Rolls Lord Woolf.
They called for an immediate stop to any new Lords appointments - with an "absolute cap" of 750 members - and for retirements to be allowed for the first time.
While the Government is committed to reducing the number of MPs from 650 to 600, the Coalition Agreement also states that "Lords appointments will be made with the objective of creating a second chamber that is reflective of the share of the vote secured by the political parties in the last general election".
Today's report said that this aim was "unrealistic" and require the appointment of at least another 269 peers.
"This would have disastrous consequences for the operation of the chamber, and be deeply unpopular with the public," it said.
"Even if this was not the case, it would be a foolish and unsustainable course to pursue, as it would result over time in a second chamber whose size spiralled ever upwards."
The report's author and deputy director of the Constitution Unit, Meg Russell, said: "It is unusual for a group of such senior figures to come together on a cross-party basis to call for change, but there is huge concern in the House of Lords about this issue.
"The fear is that David Cameron may unwittingly destroy the Lords through this volume of appointments.
"We await Lords reform, but in the meantime we must maintain a functional parliament.
"The risk is that reform fails - as it often has before - but that meanwhile the Lords has become bloated and dysfunctional."
A Downing Street spokeswoman said: "The Government will be bringing forward a draft bill before the end of May proposing a wholly or mainly elected House of Lords.
"The Programme for Government states 'In the interim, Lords appointments will be made with the objective of creating a second chamber that is reflective of the share of the vote secured by the political parties in the last general election'.
"The current system of appointing peers will therefore remain until the Government's reforms are in place."