Members of the House of Lords who did not vote in the last parliamentary session claimed £100,000 in expenses, a report has found.
From 2010-15 a total of £360,000 was claimed by 62 peers who did not vote, the Electoral Reform Society said.
Members are not paid a salary but can claim a daily allowance of £300 if they attend a sitting.
A spokesman for the campaign group said the statistics showed the House of Lords was “growing out of control”.
Darren Hughes, the group’s deputy chief executive, added: “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.”
The report – entitled House of Lords: Fact vs Fiction – also found that 10 peers were responsible for claiming £260,000 of the £360,000 from 2010-2015.
The society estimates that if the prime minister forges ahead with plans to appoint 50 additional members of the Lords it will cost at least £1.3m per year.
The report also says that more than a third of Lords previously worked in politics, and more than half were over 70 years old, while only two were younger than 39.
Mr Hughes said: “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“This is a shockingly out of date and unrepresentative institution.”
Who’s in the House of Lords?
currently eligible to vote in the House of Lords
670 Life peers
87 Hereditary peers
£300 daily expenses available to each peer for attending at Westminster