Two ex-prostitutes have dropped a civil suit against former IMF chief Dominique Strauss-Kahn, who is standing trial for pimping in France, their lawyer says.

The former prostitutes had delivered dramatic testimony during the trial of how they felt powerless to say no to Strauss-Kahn sodomising them during sex parties.

But their lawyer said on Monday that they felt unable to prove that the former political heavyweight knew the women were prostitutes and were dropping their civil case against him.

It was running in parallel to the criminal trial in which Strauss-Kahn and 13 others are charged with pimping and could face up to 10 years in prison.

The trial in the northern city of Lille has entered its third and final week, with lawyers and prosecutors set to sum up their cases after a fortnight of often lurid accounts about the orgies.

One of the ex-prostitutes, Jade, said the forced sodomy carried out by Strauss-Kahn was evidence he knew he was with a prostitute.

"I experienced a penetration without my permission. If I was a libertine, I would at least have been asked if I wanted to do that," she told the court last week.

But Strauss-Kahn argued he was not on trial for "deviant sexual practices" and was "horrified at the practice of using prostitutes".