The Liberal Democrats have said they will block parliamentary boundary changes if rebel Tory MPs halt plans to reform the House of Lords.
Richard Reeves, a senior aide to Nick Clegg, said there would be "broader consequences" for the coalition if it failed to win a crucial vote next week on Lords reform.
Conservative backbenchers, deeply opposed to Mr Clegg's plans for a mainly-elected upper chamber, are threatening to combine with Labour MPs to kill off a programme which would allow the Government to push through this legislation in the Commons, with limited debate on the matter.
Mr Reeves admitted if no time limit were to be set on the debate over Lords reform, the chances of the Bill reaching the Statute Book were "vanishingly small" as opponents would be able to talk out the proposals.
He made clear that if that happened, the Lib Dems would block plans to redraw parliamentary boundaries and cut the number of MPs from 650 to 600 - a measure which some experts believe could give the Tories an additional 20 seats at the next general election.
"It is a very serious moment for the Government," Mr Reeves said.
"There would be broader consequences for the Government's programme, particularly around political and parliamentary reform."