David Cameron's plans to appoint more Conservative peers to the House of Lords could cost taxpayers millions of pounds, the Electoral Reform Society has warned.
The warning came amid fresh calls for reform following controversy over peers' expenses claims, and just days after Lord Sewel resigned his seat after being filmed apparently taking cocaine with prostitutes.
The Prime Minister yesterday said there was "no point" in reviving Lords reform after it was blocked by Tory backbenchers in the last Parliament.
And he indicated he plans to use the upcoming dissolution honours to redress the balance in the Upper House - where 226 Tories are heavily outnumbered in a chamber of 790 peers - so that it more closely reflects the state of parties in the Commons.
The ERS said reported plans to appoint 50 more Tory peers over the summer would cost at least £1.3 million in expenses and allowances, while an additional 100 peers over the course of the Parliament would cost £2.6 million.
Actually bringing the distribution of seats in the Upper House in line with the Commons would require an additional 704 Tory peers, unless large numbers of existing Lords were kicked out, the ERS calculated. This would bring the House - already the largest legislative assembly outside China - up to 1,490 members and cost at least £18 million.
House of Lords accounts show £93 million net operating costs in 2013/14 - or £118,000 for each peer - said the ERS. In the 12 months from February 2014, £21,424,729 was spent on allowances and expenses, with the average peer receiving £25,826.
ERS deputy chief executive Darren Hughes said it was "frankly shocking" that Mr Cameron was considering adding more peers: "These latest figures only serve to reinforce the need for a radical rethink about our second chamber, which is getting bigger and more expensive by the day.
"It can't be right that when politicians are talking about reducing the cost of politics, they're set to stuff the upper chamber with yet more party appointees. We urgently need to sort out the House of Lords and move to a fully-elected chamber where the people who make our laws are elected by the public - and can be kicked out by the public."
Controversy over the cost of the Lords was fuelled by Daily Mail research showing that 124 peers have claimed a flat-rate £300-a-day to attend Westminster despite living in London.
Labour MP John Mann said the situation was "a scandal as big as the MPs' expenses scandal", with no transparency over whether peers actually do any work after signing in to claim their allowance.
A former chair of the House of Commons Political Reform Committee, Graham Allen, said it highlighted the need for change to ensure the Upper House properly reflects the nations and regions of the UK.
"If we were allowed to elect Lords by nation, there would be an expectation, quite rightly, that those people should have proper arrangements for travel to and accommodation near their place of work, rather than just having the House attended by rich people who live handily for Westminster," he said.
And Scottish National Party MP Pete Wishart said: "It is ridiculous that Westminster has unelected peers claiming up to £300 a day just to clock in. With Tory austerity set to continue, Westminster parties need to review the system."
A House of Lords spokesman said that the daily allowance, introduced in 2010, does not relate to the cost of travelling to Westminster, which can be claim separately as an expense by peers living outside London.
"The allowance introduced in 2010 is based on attendance, not residence," he said.
"Members of the House of Lords are not paid a salary. They can claim £300 or £150 for every day they attend the House and undertake parliamentary work.
"From this allowance, members are expected to pay for any costs associated with attending the House such as their London accommodation, as well as staff cost."