The government Chief Whip in the Lords, Lord Carter, is said by senior sources to have told the Prime Minister a hunting Bill could still be blocked by a coalition of Tory, crossbench, hereditary and Liberal Democrat and Labour rebel peers.
Mr Blair will face protesters from pro-hunt and anti-hunt camps at Labour's conference in Bournemouth next week. The warning the Government cannot guarantee early delivery of his promise of a ban will disappoint party activists.
The British National Party is reported to be planning to infiltrate the protests by campaigners, who expect a turn-out of about 18,000 in Bournemouth.
Jack Straw, the Home Secretary, is believed to be working on a government Bill to ban fox-hunting with dogs, but it may be necessary to invoke the Parliament Act to force it through the Lords, delaying it for 12 months.
Michael Foster's previous attempt to bring a backbench Bill was "talked out" before it reached the Lords, despite passing its second reading in the Commons with an overwhelming majority. Mr Foster accused pro-hunting MPs of "abuse" of the Commons.
Leading Liberal Democrats doubt Mr Blair will want Parliament to become entangled with a row over fox-hunting in the run-up to a general election.
Liberal Democrat peers say they will be split if the Government goes ahead with a Bill. Mr Blair forced the issue on to the agenda by committing the Government to action on BBC1's Question Time programme.
Mr Straw studied options, including allowing local authorities to hold referendums on local bans, or a national ban with opt-out referendums. Campaigners were told the Government has dropped the proposals and is still wrestling with its programme for the next session.