The UK coalition government is under its most acute strain since the partnership of unlikely allies was formed in 2010, Deputy Prime Minister Nick Clegg has acknowledged as he confirmed his cherished hope of reforming the House of Lords has been doomed by opposition from his own colleagues.

Clegg, leader of the Liberal Democrats – the junior member of Britain’s coalition – said on Monday resistance from Prime Minister David Cameron’s Conservative Party had killed the chance of passing a flagship bill to reform Britain’s unelected upper chamber.

Reforming the chamber was a key ambition for Clegg – who cited the chance to push through the changes as a reason for aligning with Cameron’s Conservatives to form the country’s coalition after the 2010 election ousted the Labour Party government, but saw no single party gain a majority.

“An unelected House of Lords flies in the face of democratic principles and public opinion. It makes a mockery of our claim to be the mother of all democracies,” said a disappointed Clegg, who conceded there was no prospect of reviving the plan before Britain’s scheduled 2015 election.

Plans announced in the government’s latest legislative slate had envisaged transforming the House of Lords into a mainly elected 462-seat chamber by 2025.

The 700-year-old body – which can amend planned laws, but has no role in creating legislation – currently has about 775 working members, a mix of 660 political appointees, 89 hereditary peers – who inherited a place in the chamber from their nobleman forebears – and 26 people who hold ecclesiastical offices, like the Archbishop of Canterbury Rowan Williams.

The disagreement over Lords reform has highlighted the differences between the centre-right Conservatives and Clegg’s often politically left-leaning Liberal Democrats.

Their alliance was regarded as unlikely, and tensions have surfaced over their approaches to Europe, Britain’s voting system and response to the tabloid phone hacking scandal.

Clegg insisted that his party would not abandon the coalition government – which could potentially trigger an election earlier than 2015 – but he accused Cameron’s party of reneging on the deal the leaders struck in 2010.

“The Conservative Party is not honouring the commitment to Lords reform and, as a result, part of our contract has now been broken,” Clegg said.

In response, Clegg’s Liberal Democrats will vote against Cameron’s plans to overhaul Britain’s electoral map – a move that would reduce the size of the House of Commons by 50 MPs and create more evenly sized voting districts.

Analysts predicted the planned changes would have seen Cameron’s party claim about an extra 16 to 20 seats, potentially enough of an advantage for his party to win an election outright and avoid governing in coalition in the future.