THE French national assembly has voted to tax the highest earnings in the country at a rate of 75 per cent, a campaign promise made by President Francois Hollande that has been criticised by conservative politicians.

The measure would affect earnings of more than 1 million euros ($A1.27 million) per year and is supposed to last for only two years.

It would be paid by an estimated 1500 people and provide the government with an extra 210 million euros in revenue per year.

Budget Minister Jerome Cahuzac termed the proposal "legitimate" and insisted it was not a question of simply confiscating hard-earned money.

"Each of us must contribute according to their means" to a collective effort to correct France's financial situation, which includes a heavy debt load, Cahuzac said.

He stressed that the two-year period was the time it should take to straighten out the French public deficit.

The French budget rapporteur, Socialist Christian Eckert, explained that the tax was meant to be "dissuasive" and added that what was targetted were exceptional pay packages that have fueled howls of protest in the country.

"What we want is that these practices disappear," Eckert said.

But conservative politician Eric Woerth from the UMP party said: "75 per cent is a punitive tax."

He underscored that "we are going to heavily tax a small category of people, which will bring in very little and undoubtedly cause some (taxpayers) to leave" the country.

Centrist politician Charles de Courson said his UDI party would ask the French constitutional council to examine the bill to determine if it violated laws on equal treatment.