An Italian court has opened an investigation on vice-president of the Senate Roberto Calderoli for saying that the country's first black government minister has "the features of an orangutan".

Calderoli, a prominent member of the anti-immigration Northern League party, addressed his supporters in Treviso with a racist slur addressing Congo-born Cécile Kyenge, an Italian citizen who is minister for integration in Enrico Letta's government.

"I love animals... but when I see pictures of Kyenge I cannot but think of - even if I'm not saying she is one - the features of an orangutan."

Prosecutors in northern Italy's city of Bergamo are now investigating Calderoli for slander with racial hatred.

Calderoli, who said he would not quit over the racist jibe, apologised to Kyenge but refused to call his comments a racial slur. He insisted he was only joking when he made the "aesthetic" comment about the minister.

"I recognise that it was an unfortunate joke. I apologise to the minister but I'm accustomed to comparing people to animals," he told daily La Repubblica.

"When I see Prime Minister Enrico Letta, with his long legs, I think of a heron. Angelino Alfano [minister of interior] resembles a frog while justice minister Anna Maria Cancellieri looks like a St. Bernard dog."

Northern party leader Robert Maroni said the issue was "closed".

Kyenge, who has faced daily racial threats since she was appointed in April, played down the row but said that if Calderoli could not translate his views into proper political discourse he should perhaps step aside as the Senate's vice-president.

In an interview with daily Corriere della Sera, Kyenge said racism is a "threat to the country's international image".

"This is a moment to stop and reflect together on the image of Italy tomorrow," she said.