Yusuf al-Qaradawi, an Egyptian who has lived in the Qatari capital Doha for several years, said: "Measures like banning the headscarf will feed extremism."
"If the law is passed, we will seek to file a legal complaint because this law will be in contradiction with the French constitution," al-Qaradawi said.
He added that the European Council for Fatwas and Research, which he chairs, had called on France to revise its position on the hijab.
A council delegation, led by Mauritania's former justice minister Abd Allah bin Baya, is to leave for France to discuss the issue.
Last month, al-Qaradawi asked French President Jacques Chirac to "go back on his decision" and said in a letter addressed to the French ambassador in Qatar he was saddened by the proposed ban.
Al-Qaradawi's missive condemned "this unrelenting attack on the precepts of Islam by France, a country of liberty and tolerance".
On 17 December Chirac gave his approval to a plan to ban the hijab and other "conspicuous" religious symbols in state schools.
He wants the rules written into law by the start of the next academic year.
The decision, intended to reflect France's strict separation of religion and state, has set off a storm of protest by Muslim leaders around the world.
Al-Qaradawi is a regular guest on Aljazeera's popular programme, Al-Sharia wal-Hayat, where he answers viewers' questions pertaining to the Islamic faith.