The number of peers in the House of Lords could soar from 756 to around 2,000 after the next election, leaving unelected lawmakers outnumbering elected MPs by three to one, a report by the Electoral Reform Society has concluded.

The huge increase – the opposite of what was intended when plans to reform the second chamber collapsed last year – would come about if people vote in line with recent opinion polls and install a Labour government, while punishing the Liberal Democrats at the ballot box. The report, The Super Sized Second Chamber, says that in this particular scenario the number of peers in an unreformed Lords would need to increase to around 2,000 in order to reflect the new political balance in the Commons.

The projections are likely to reignite political debate about Lords reform, which is already simmering after lobbying controversies involving Lords Cunningham, Mackenzie of Framwellgate, and Laird. The three were caught up in "sting" operations by Sunday Times reporters posing as lobbyists, raising new questions about the behaviour of some who are elevated to the second chamber. Peers are able to take a tax-free allowance of £300 a day for attending the upper house, though they do not have to attend debates or vote.

The ERS projections assume that the Liberal Democrats' vote falls to 10% in 2015 from 23% in 2010, with Labour polling 38%, the Tories 31% and other parties, including Ukip, 21%. If the House of Lords were to be adjusted to reflect that political balance after 2015, it would need 437 more Labour peers and 264 more Tory peers, plus 271 new peers from other parties. Were Ukip to be fairly represented, Nigel Farage's party would need 273 peerages.

Darren Hughes, director of campaigns and research at the ERS, said the huge potential increases were a result of the inability to get rid of Liberal Democrat peers – whose number by then could have risen to 172 – around whom the rebalancing would have to take place. "We are sleepwalking towards a super-sized second chamber," said Hughes. "Nearly every poll is showing the Liberal Democrat vote share in the low teens. That will require the next prime minister to rebalance the Lords around an immovable block of Lib Dem peers."

The way to avoid this explosion in the number of peerages, Hughes suggested, would be for Miliband to make clear now that he would change the system in Labour's first term — something that would be easier said than done given the inevitable opposition from many peers themselves.

Hughes said: "Labour have remained tight-lipped on constitutional policy. There is an opportunity here to show some leadership. Ed Miliband needs to decide now if, come 2015, he intends to pack the house to the rafters or bring the curtain down.

"This is a prime example of where Labour and Liberal Democrats could be working together to make some progress. If they came up with a plan before the election it would knock vested interests for six when they try their stalling tactics.

"It's too easy to say Lords reform is not a priority, but if Labour maintain their poll lead this will be in PM Ed Miliband's in-tray marked 'urgent'. Otherwise we are looking at a future where appointed cronies outnumber elected politicians on the national stage by three to one."

Most peerages are created following the dissolution of parliament after the calling of a general election (the dissolution honours list) and the resignation of a prime minister (the resignation honours list). Some are also announced in the new year and the Queen's birthday honours lists.

The prime minister's latest appointments are expected to take the chamber to 876 by the end of 2013.