Britain's The Sun tabloid reported that Blair would leave office on July 26, 2007 - after 10 years as Britain's premier. Two ministers from Blair's Cabinet said that next year was likely to see him step down.
The newspaper's Wednesday edition, available late on Tuesday, splashed across its front page that Blair would resign as leader of the governing Labor party on May 31, 2007 - triggering a leadership election likely to take around eight weeks. He would then be replaced as Prime Minister on July 26, the tabloid said.
Blair was appointed Labor leader in July 1994 and took office on May 2, 1997.
George Pascoe-Watson, political editor of The Sun, told Britain's Sky News television he would not disclose the sources of the story, and said only that his newspaper's previous predictions on the date of elections had been "absolutely bang on the money."
Britain's Daily Mirror tabloid said in its Tuesday edition it had been leaked a copy of the purported memo setting out the details of Blair's leaving plans - but not any departure date.
Lawmakers from the party were also circulating a letter demanding he set a resignation date, news reports said.
Pressure from within Labor for Blair to publicly set a time frame for his departure has intensified recently. Critics worry that the uncertainty over when he will leave is damaging the party's electoral hopes, while Blair's waning popularity is giving the resurgent Conservative Party an opening.
Blair last week shrugged off demands that he announce his plans at the party's annual conference later this month.