The Home Secretary's remarks provoked an angry response from Charles Kennedy, Liberal Democrat agriculture spokesman, who said his party's co-operation with the Government on constitutional affairs could be broken off if the Cabinet rejected the Jenkins report, due next month.
"If Tony Blair is to turn round and say, `Very interesting report, but I can't accept it, end of story', then I think obviously that would be a massive rebuff not just to the Liberal Democrats, but for Roy Jenkins himself," he told BBC TV.
"It would be very difficult to see then how you could re-fashion a new relationship when perhaps the most important aspect of it, which was fundamental reform of the British political system, that opportunity had been jettisoned by the Prime Minister himself."
Mr Straw also cast fresh doubts over the Government's manifesto commitment for a referendum on PR, refusing to confirm it would be held before the next election.
The doubts over PR make it more difficult for Paddy Ashdown, the Liberal Democrat leader, to keep his party in check this week at its annual conference in Brighton.
Mr Blair faces troubles within his Cabinet, and is under pressure to drop collective cabinet responsibility to allow senior colleagues to campaign for a "No" vote in the referendum on electoral reform. His official spokesman said the Cabinet had not yet considered whether cabinet collective responsibility would be enforced.
John Prescott, Deputy Prime Minister, opposes PR, and at least five other cabinet ministers are expressing doubts, raising the possbility that Mr Blair may allow cabinet members to express their own views on the constitutional issue.
The Jenkins commission is widely expected to recommend a system called "Alternative Vote-Plus". That would mean about 500 seats would be elected by alternative voting, but 150 would be allocated proportionately from party lists.
There is a growing belief that Mr Blair will accept it as a workable compromise but boundary change delays would mean the next election would be fought on the existing system. Lib Dem conference, page 8
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