The development has cast added doubt on whether the EU's negotiating position remains united.
French diplomats said Chirac told 24 EU presidents at an EU summit in Hampton Court, England, on Thursday that "France reserves the right not to approve at Hong Kong an agreement" to liberalise global trade - including agriculture, a key sticking point.
The diplomats, speaking on condition of anonymity, also said Chirac told the summit the EU "should not put into question" a previously negotiated farm reform package agreed upon in 2003.
France has been at loggerheads with EU Trade Commissioner Peter Mandelson and the European Commission, claiming it has gone too far in offering too many concessions in trade talks at the World Trade Organisation.
Eye on Hong Kong
The European Commission is negotiating on behalf of the 25-nation bloc at the world trade talks, which will hold a key meeting in Hong Kong in December.
The 148-country WTO remains split on a new trade pact that aims to increase market access for poorer countries, to boost the opening up of services sectors and to reduce farm subsidies.
On Friday, senior EU trade officials are to meet in Brussels to address France's concerns. Chirac said he wanted clarification at that meeting.
Trade czar Mandelson (L) has the
support of most EU members
Mandelson is putting together a new offer on farm trade to be part of the global trade negotiations.
In recent weeks, France has argued that Mandelson has exceeded his negotiating mandate by offering cuts in subsidies and import tariffs that go beyond what the 25 EU governments had approved.
Mandelson has forcefully denied that and has to date retained the support of the majority of EU nations, but any move to alleviate Chirac's concerns could undermine Mandelson's negotiating position.