The money came from several presidents of France's former African colonies, and was handed over by himself to the centre-right politicians in stages between 1995 and 2005, Robert Bourgi said in an interview with Europe 1 radio.
Insisting he was coming forward now because he wanted a "clean France", Bourgi said the system of kickbacks had also existed under former presidents Georges Pompidou, Valery Giscard d'Estaing and Francois Mitterrand.
He said he could not estimate how much had been handed over before he became directly involved, but could speak about the deliveries he said he had made to Chirac's office when he was mayor of Paris and later to Villepin.
"I'd estimate at around $20 million what I handed to Mr Chirac and to Dominique de Villepin," he told Europe 1, fleshing out the detail of claims he had already made in a newspaper interview that appeared on Sunday.
The allegations, which were furiously denied by Chirac and Villepin, come just seven months before France's presidential election, in which President Nicolas Sarkozy could face a Villepin challenge from within the right.
Villepin, a suave diplomat best remembered for leading the charge against the Iraq war at the United Nations in 2003, has said the revelations are aimed at derailing his presidential bid.
Bourgi, an unofficial long-time pointman between French and African leaders, catalogued what he said were lavish gifts bestowed by African rulers on their counterparts in Paris, including memorabilia to noted Napoleon Bonaparte fan Villepin.
On Sunday, Bourgi detailed other gifts, including a watch with 200 diamonds given to Chirac by Bongo.
"A splendid object but difficult to wear in France," Bourgi said.
Bourgi also alleged other French politicians took African cash.
On Monday he claimed in a French television interview that the former head of the far-right National Front party, Jean-Marie Le Pen, financed part of his 1988 presidential campaign with money from Gabon's ex-president Omar Bongo.
Le Pen denied the claims, saying they were "false... ridiculous," and said he would file a defamation complaint against Bourgi.
"If this Mr. Bourgi was charged with giving me these funds, he must have kept them for himself," Le Pen told BFM television, adding that he thinks Bourgi is being guided by Sarkozy.
Bourgi is widely reported to be close to Sarkozy, but insisted in the interview that he was neither an official nor unofficial advisor to the president, simply someone who was sometimes consulted for an opinion.
While Bourgi has been at pains to stress that such practices did not continue under Sarkozy, who gave him the prestigious Legion d'Honneur award in 2007, a former aide to Chirac said on Monday that "nothing stopped with Sarkozy".
"Bourgi worked tirelessly for Sarkozy with numerous African heads of state during the 2007 presidential election," Jean-Francois Probst told Le Parisien daily.
He "went to Libreville as early as July 2007 and made a new deal with Omar Bongo, who, they say, gave him a billion African francs (over €1.5 million)", Probst said.
The suggestion that payments had stopped under Sarkozy is "not credible, it's even the biggest lie of his life", Probst said.
An advisor to former Ivory Coast president Laurent Gbagbo, Bernard Houdin, said that such payments were "a historical practice" and that "the sums mentioned are no doubt below reality".
Chirac and Villepin have said they will lodge lawsuits against Bourgi, who described himself as "repentant" after having for years taken part in "the dark side of Francafrique", a French term describing the country's sometimes murky relations with its former colonies in Africa.
Asked what African leaders got in exchange for their cash, Bourgi said:
"Lies, lies, lies, unkept promises, meaning that France would close its eyes to certain abuses of power in Africa."
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Bourgi admitted that he had no proof to back up his allegations, while a former minister from Sarkozy's UMP party, also a lawyer, said he was shocked by the revelations as they implicated Bourgi himself.
"I'm surprised because Mr Bourgi is seriously incriminating himself," Patrick Devedjian told LCI television.
"He will have to give a serious account to justice, beyond defamation."
The Paris bar association said on Monday it had opened an ethical inquiry into the truth of the missions that lawyer Bourgi said he carried out.
Burkina Faso meanwhile rejected as "grotesque" allegations that its president was among African leaders who gave money for Chirac's election campaign.
"It is an absolutely grotesque statement by someone who ... wants to crucify his former friends because of problems of positioning in French politics," Alain Edouard Traore, a spokesman for the government of President Blaise Compaore, told journalists.
"I really don't see what we, Burkina Faso, have to do with this French story. Let them sort out their own problems," he said.