The biggest catch for Macron was the defection of Defence Minister Jean-Yves Le Drian, a Socialist Party grandee who has been both a close ally and friend of outgoing President Francois Hollande for nearly 40 years.

The official Socialist candidate Benoit Hamon, whose low poll rating suggests he will be eliminated in the first round of voting in the presidential election on April 23, bitterly accused Le Drian of short-changing his own voters.

"In democracies, it is not acceptable that politicians only honor the will of those who elected them when it suits," Hamon said in a statement.

Macron, a political novice who has never held elected office, has gatecrashed the leadership contest with a pledge to transcend the long-established divide of Left-versus-Right politics with an program for cross-partisan government.

His position was confirmed by a Harris Interactive poll of more than 6,000 voters which showed Macron coming first in a first round of voting and then trouncing far-right leader Marine Le Pen in the two-way runoff on May 7.

The second defection was that of Thierry Braillard, a junior sports minister.

Earlier this week, biodiversity minister Barbara Pompili, another junior member of the Socialist leadership which has been in power since 2012, also switched allegiance to Macron rather than back Hamon. The latter has been fourth-placed in the polls for weeks and fell to fifth spot in the Harris poll.

Only the two top candidates in the April 23 opening round, which all opinion polls say will be Macron and Le Pen, can go through to the May 7 runoff.

Hollande, the first president not to seek re-election since the introduction of universal suffrage in 1962, had appealed to ministers to refrain at least until the end of this week from taking pre-election positions.

Le Drian announced his move to Ouest-France, a regional newspaper. The support of a respected voice in security matters and foreign affairs is particularly welcome news for the youthful former economy minister, who has been criticized by opponents for lacking experience in these areas.

Braillard went public with an announcement on Twitter and an RTL radio interview in which he said: "Emmanuel Macron's program is the one that best fits the challenges facing France."

Macron also won support from the other side of the political divide. Philippe Douste-Blazy, a right-wing former minister for health and foreign affairs, told Marianne magazine he would back him. Dominique Perben, a right-winger who was minister for justice and transport in the past, did likewise on Wednesday.

The campaign so far has been dogged by judicial inquiries into two major candidates -- Francois Fillon, a conservative former prime minister who was frontrunner until he was engulfed by allegations of financial wrongdoing, and Le Pen over European Parliament funds she paid to her bodyguard and chief secretary.

As a result of a scandal around payments of public funds to his wife and children for allegedly little work as parliamentary assistants, Fillon, a fan of the late British leader Margaret Thatcher, faces elimination after falling to third place in the polls.

He, like Le Pen, denies any wrongdoing but both are under official investigation.