But his condition stabilised after treatment including "respiratory assistance", the Vatican said.

It added that the Pope, who was last seen in public on Sunday, was able to take part in Mass on Wednesday morning.

The Polish-born pontiff is also suffering from Parkinson's disease and painful joint conditions.

"I think everyone has to be calm because there is no reason for alarm today," spokesman Joaquin Navarro-Valls told reporters outside the hospital on Wednesday morning.

The Pope was forced to cancel all engagements for several days after coming down with flu on Sunday, although he made his regular midday appearance at his window overlooking St Peter's Square.

The pontiff will miss his weekly audience on Wednesday for the first time since September 2003, when he had an intestinal ailment.

The Pope was taken by ambulance to the Gemelli hospital at about 2300 local time (2200 GMT) on Tuesday.

"The flu which has been affecting the Holy Father for three days was complicated with acute inflammation of the larynx and laryngo-spasm," Mr Navarro-Valls said in statement at the time.

Laryngo-spasm is a closure of the larynx that blocks the passage of air to the lungs - a condition where one cannot catch one's breath.

The Pope is being cared for in the same room at the hospital where he has been treated several times before.

It is the same hospital where he recovered from an attempt on his life in 1981, when he was shot in the abdomen.

Vatican sources say a contingency plan was made for the Pope to be taken to hospital if he fell victim to the current flu epidemic sweeping Italy. A set of rooms is always reserved for him on the 10th floor.

The appointment of Karol Wojtyla as the first Polish Pope in 1978 was in many ways seen as a groundbreaking move for the Roman Catholic Church.

The first non-Italian Pope in 455 years - and at 58, the youngest Pope of the 20th Century - he was little known outside Vatican circles, and few experts tipped him as successor to Pope John Paul, who died after only 33 days in office.

John Paul II is now in the 27th year of his pontificate which makes him almost the longest-reigning Pope in history.

A 19th Century Pope occupied St Peter's throne for more than 30 years and no-one knows exactly how long the first Pope, St Peter, lived.

John Paul II - the leader of the world's estimated one billion Catholics - has visited more than 100 countries, and is estimated to have effectively circled the globe 27 times.