John Prescott, the Deputy Prime Minister, has rejected calls for a "substantially" elected House of Lords, despite claims that ministers will agree a compromise to quell the anger of Labour backbenchers.
Government plans for only one in five members of the new upper house to be elected have been rejected by Labour backbenchers, who want at least half of the members to be chosen at the polls.
Robin Cook, the Leader of the House of Commons, has hinted at a compromise, by insisting ministers will find a "centre of gravity" on the issue.
But Mr Prescott told The Independent on Sunday: "If we move away from 20 per cent, it is a choice between nothing and 100 per cent.
"A wholly elected House of Lords is totally unacceptable. Some are saying they are looking for a new centre of gravity which talks about a substantial proportion being elected. That would be a direct threat to the sovereignty of the House of Commons. It would also change the role of the House of Lords as a revising chamber.
"My centre of gravity is a choice between 100 per cent and zero, and if the option of 20 per cent is not acceptable, I would like to reassert the option of zero," he said.
The Government's plan for a 600-member House, with 120 members directly elected, 120 appointed by an independent body and the rest nominated by political parties, has been savaged by MPs and peers.
Conservatives and Liberal Democrats have called for at least 80 per cent of the new House to be elected, and say a majority in the House of Commons favours radical reform.
Mr Prescott's comments follow reports that the proportion of peers elected may rise to 50 per cent in phases over a number of years. Ministers have promised a "period of reflection" before any new proposals.