The Socialist favourite in France's presidential election, Francois Hollande, has said top earners should pay 75% of their income in tax.
"Above 1m euros [£847,000; $1.3m], the tax rate should be 75% because it's not possible to have that level of income," he said.
Speaking on prime time TV, he promised that if elected, he would undo tax breaks enacted by Nicolas Sarkozy.
The tax proposal was condemned by his political opponents.
Opinion polls suggest the gap between the Socialist candidate and Mr Sarkozy has narrowed.
The two are tipped to reach the run-off on 6 May, after eliminating other rivals on 22 April.
Taxation for the rich has become a hot campaign issue, with tax advisers in neighbouring Switzerland saying that higher taxes for the wealthy in France could spark an exodus, Reuters news agency reports.
Many of France's richest celebrities already live abroad.
The French right-of-centre newspaper Le Figaro reports that Mr Hollande's announcement on the TF1 channel appeared to take party colleagues by surprise.
Jerome Cahuzac, responsible for budgetary affairs on Mr Hollande's campaign team, was questioned about the 75% rate on another channel, France 2, just minutes afterwards.
"You are asking me about a declaration which, for my part, I haven't heard," he said.
Mr Hollande himself renewed his call on Tuesday, saying the 75% rate on people earning more than one million euros a year was "a patriotic act".
"It's a signal that has been sent, a message of social cohesion, there is an effort to be made," he explained.
"It is patriotic to agree to pay a supplementary tax to get the country back on its feet."
Centrist presidential candidate Francois Bayrou dismissed the idea.
He told another TV channel, BFMTV: "I think it was [French film director Michel] Audiard who used the rather rough phrase: the rubbish-ometer [French: deconnometre] is working overtime."
Ministers from Mr Sarkozy's ruling UMP party also attacked the proposal.
Francois Hollande "invents a new tax every week without ever proposing the smallest saving", said Budget Minister Valerie Pecresse and Foreign Minister Alain Juppe denounced the plan as "fiscal confiscation".
When Mr Sarkozy came to power in 2007, he introduced a "tax shield" that capped tax at 50% of all income.