Liverpool manager Gerard Houllier was tonight undergoing cardiac surgery, according to a statement released by the club.
Houllier was taken to hospital during Liverpool's Premiership match against Leeds United this afternoon after experiencing chest pains.
He had a series of tests which revealed that he had not suffered a heart attack.
But the tests did reveal a problem requiring the 54-year-old Frenchman to undergo cardiac surgery.
Liverpool's statement read: "Gerard Houllier did not suffer a heart attack earlier today, but, as a result of tests carried out at Royal Liverpool and Broad Green hospitals, Gerard is now undergoing cardiac surgery.
"The club will be making no further comment until that surgery is completed, but we underline that Gerard could not be in better hands.
"Gerard will definitely be out of action for several weeks and during that period Phil Thompson will take charge of team matters while the manager recovers."
A club source confirmed that Houllier was laughing and chatting with nurses and doctors in hospital.
But it is clear that there is no such thing as minor heart surgery and Houllier will clearly need time to rest and recuperate after his surgery.
Liverpool were not prepared to reveal the exact nature of the surgery Houllier was undergoing.
It means that Houllier will be sidelined for arguably the most important part of Liverpool's season leading up to Christmas.
They travel to Kiev tomorrow for Tuesday's Champions League clash with Dynamo Kiev, and Thompson will take control for the first time for that match.
But the club have two more Champions League group matches in the next three weeks _ away to Boavista and at home to Borussia Dortmund _ plus a home Premiership clash with Manchester United.
The need to qualify for the second phase of the Champions League is paramount and they dare not fall behind in the title race.
Houllier will be badly missed, for his calm control, tactical ability and leadership, and the mantle falls on Thompson to carry the club through a very demanding period.