PARIS (AFP) - The trial of Franck Ribery and Karim Benzema for having sex with an under-age prostitute opened Tuesday but was immediately suspended pending resolution of a point of law which may result in charges against the French football stars being dropped.

Lawyers for Ribery, 30, and Benzema, 25, challenged the legal basis for the charges against their clients and the judge agreed that they had raised a legitimate question which will have to be addressed by another court before the trial can continue.

Neither Bayern Munich winger Ribery or Real Madrid striker Benzema attended Tuesday's brief proceedings. Also absent was Zahia Dehar, the former call girl turned fashion designer at the centre of the case.

The two players are accused of having paid for sex with Dehar when she was 16, in Benzema's case, and 17, in Ribery's.

Although the age of consent is 15 in France, paying for sex with anyone under 18 is a crime punishable by a three-year prison term and a fine of up to 45,000 euro (S$75,000) fine.

The players' lawyers argued that the law has been drafted imprecisely and may be unconstitutional since it could permit a conviction in a case where the client could not reasonably have known the prostitute was under 18.

That issue will now be reviewed by a Court of Cassation which can either order the trial to continue or refer the issue to France's Constitutional Council for a ruling.

Each body has a maximum of three months to deliver a ruling and the trial will resume on January 20, 2014.

Ribery has admitted having sex with Dehar after she was flown to Munich in 2009 as a "present" for his 26th birthday, but insists he could not have known she was not 18 at the time.

Benzema, who is accused of paying for sex with Dehar in a Paris hotel when she was 16, denies that any encounter took place.