Berlin - Polar bear Knut celebrated his second birthday on Friday blissfully unaware that he might soon have to leave his home at Berlin Zoo.

Hundreds of visitors broke into a chorus of Happy Birthday to You as the zoo's star attraction munched into a cake of ice cream, fruit and vegetables topped with two leeks symbolizing birthday candles.

Before that Knut made short work of his favourite food, croissants, and then tucked into a large chuck of salmon.

Fans brought birthday cards, flowers and gifts of apples and nuts as Knut frolicked in the overcast winter weather, visibly enjoying the attention he was getting.

But the party was overshadowed by fears that the polar bear might soon have to leave Berlin because he is getting too big for his enclosure.

Knut won global fame after being hand-reared by his keeper for 108 days before becoming too big and dangerous to play with humans.

More than five million visitors have come to Berlin to see Knut, earning the zoo 5 million euros alone last year from the sale of tickets, toys and other merchandise.

On the eve of his birthday, the zoo where Knut's father comes from restated its claim to ownership of the bear. Peter Druewa, director of Neumuenster Zoo, north of Hamburg, said Knut would leave Berlin.

Another zoo in the western German city of Gelsenkirchen has expressed interest in having Knut as a partner for its female polar bear, Lara.

Animal lovers have collected 21,000 signatures in a petition for Knut to remain in Berlin, while the city's governing mayor, Klaus Wowereit, said he does not want to see the bear leave.

Knut weighed a tiny 810 grams at birth but has now grown to a massive 200 kilos.

His keeper, Thomas Doerflein, raised Knut by bottle after the cub and his twin brother were rejected by their mother Tosca when they were just days old. His brother died after just four days.

Doerflein himself died in October, aged 44, apparently after suffering a heart attack. (dpa)