Peers claimed £360,000 in attendance fees and expenses over the last five years even though they failed to take part in House of Lords votes, new research has found.
The Electoral Reform Society (ERS) said just 10 members of the upper chamber were responsible for claiming £236,000 of the expenditure.
The body, which is campaigning for a reform of the House of Lords, warned the institution was "growing out of control" and was "shockingly out of date and unrepresentative".
Peers are able to claim £300 for each sitting day they attend the House as well as limited travel expenses.
The ERS analysis found that during the 2010-15 parliament £360,000 was claimed by peers in years they failed to vote once.
In the last parliamentary session alone, more than £100,000 was claimed by peers who did not vote at all, the report said.
The ERS said reported plans by David Cameron to appoint 50 more peers over the summer would cost at least £1.3 million a year in expenses and allowances.
It said any attempt to rebalance the upper chamber strictly in line with the 2015 general election results would require the appointment of an additional 723 members, resulting in a "supersized" upper chamber.
The group also questioned the independence of peers and the outside expertise they were able to bring - pointing out that the attendance rate of non-party crossbenchers was lower than that of party appointees.
ERS deputy chief executive Darren Hughes said: "We have shown that far from being a bastion of independence, non-partisan crossbench peers turn up far less frequently than party-political peers. And it's those peers who vote as a bloc, with Labour voting against the last coalition government, Conservative peers voting against the last Labour government nearly 100% of the time.
"On top of that, we have found that over a third of Lords previously worked in politics - compared to less than 1% of the British public.
"This is not a chamber of experts - it's a chamber of professional politicians. Our House of Lords looks nothing like the public whose decisions it impacts - almost half live in London or the South East, while there are just two peers under the age of 40. This is a shockingly out of date and unrepresentative institution.
"The Prime Minister said he 'regrets' not reforming the second House in the last parliament. It's time for him to act - and finally fix our broken upper chamber."