Former International Monetary Fund head Dominique Strauss-Kahn will be tried on charges of pimping, according to prosecutors in France.

Prosecutors in the northern city of Lille said investigating judges had determined that Mr Strauss-Kahn, 64, who has been under investigation in the case since 2012, should be judged by a criminal court.

The decision comes as a surprise after a public prosecutor had recommended in June that the inquiry be dropped without trial.

"We're not in the realm of the law, we're in ideology. We're sending someone to court for nothing," said Henri Leclerc, one of Mr Strauss-Kahn's lawyers.

The so-called "Carlton affair", named after a hotel in Lille, involves sex parties that Mr Strauss-Kahn has acknowledged attending. He said he was unaware that the women in attendance were prostitutes.

Mr Strauss-Kahn is charged with "aggravated pimping". Pimping under French law is a broad crime that can encompass aiding or encouraging the act of prostitution.

Mr Strauss-Kahn was charged with the more serious form because it allegedly involved more than one prostitute.

In France, it is not against the law to pay for sex, but is against the law to solicit or to run a prostitution business.

The crime carries a maximum term of ten years in prison and a fine of €1.5 million.

The former French finance minister quit his post as head of the International Monetary Fund in 2011 after being accused of raping a maid in New York, a charge that was dropped in August 2012.