Mayor Thomas Menino will announce Thursday that he will not run for re-election this fall.

The mayor has set an announcement for 4 o'clock at Faneuil Hall - after telling his top City Hall administrators at a meeting in the morning, sources say.

"It appears to be the end of the line," said one of the mayor's close friends, who wished to remain unnamed.

It is believed his doctors have told him the rigors of a full fledged campaign might be too much for him to withstand in his present physical condition. The mayor has apparently said that he does not want to enter a campaign and fail to go through with it.

Menino, now in the final year of his fifth term, had appeared fitter and stronger following several months of sketchy health and hospitalization and rehab. He recently moved back to his Readville home from the city-owned Parkman House.

Close associates claim he appears better than he is and that he weakens at the end of the work day.

Several sources, among them present and former elected public officials close with the mayor, said it is the people who won't call anyone back who hold the secret - spokesperson Dot Joyce, the mayor's strategist Mike Kineavy, and the mayor's best friend Mike Galvin.

"They are saying nothing because the mayor will say it all tomorrow," said a source.

Menino first broached the topic with friend in East Boston two weeks ago.

A source claims the mayor will be taking some type of position at Boston University when he leaves office in 7 months, although this could not be confirmed.

Menino, then a district city councilor from Hyde Park, became mayor in 1993: His colleagues made him city-council president, which meant he became mayor after Ray Flynn resigned as mayor to become ambassador to the Vatican.