During the news conference to publicise his long-awaited report, he said: "The prime minister's certainly not committed himself to not having a referendum in this parliament."

But the cabinet is split on the matter. Some ministers such as Leader of the House Margaret Beckett suggest the referendum could take place after the next election.

On the other hand, Foreign Secretary Robin Cook insists Labour has committed itself to a referendum during this parliament.

Downing Street has restricted itself to issuing ambiguous statements: "As the prime minister said recently, we have always envisaged there would be a referendum in this parliament."

Tony Blair's official spokesman insisted the statement was "not inconsistent" with Mrs Beckett's remarks.

The Tories are challenging Labour to name a date. Party chairman Michael Ancram says: "It is evident that Mr Blair has created something of a rod for his back by trying to curry favour with the Liberal Democrats and proponents of electoral reform whilst keeping his own cabinet, many of whom oppose PR, on side.

"In reality, his conspicuous silence and constant dithering over this important issue show that the prime minister cannot afford to move either way without breaking a partnership with one side or the other."

Internal row

The government's hesitation to commit itself to a date for the referendum seems to be inspired by a desire to prevent party splits.

Delaying the referendum may avoid an internal row for Labour but it would badly undermine Labour's pact of co-operation with the Liberal Democrats on constitutional issues. It would also put Paddy Ashdown's role as leader under pressure.