The elections on Thursday pit 53-year-old Royal, who has led polls throughout the summer, against Dominique Strauss-Kahn, a former Finance Minister, and Laurent Fabius, a former Prime Minister. One poll due to be published today will show Royal only a couple of percentage points ahead of her rivals. Newspapers are suddenly exercising a new prudence, after weeks of predicting a crushing victory for Royal. '[Relying on the polls] puts us on shifting, uncertain, unreliable and unpredictable terrain,' said Dominique de Montvalon, editor of Le Parisien.

The key issue is whether Royal can garner more than half the votes cast on Thursday and thus avoid a run-off which could see a dangerous deal between her opponents. 'If there is a second tour then we are in a whole new world of manoeuvres and deals and there is everything to play for,' said Michael Darmon, a senior political journalist and author. Some talk of an 'Anyone But Segolene' alliance.

Royal, a mother of four and the president of the Poitou-Charentes region, has suffered in a series of carefully choreographed televised debates between the three candidates. The confident and smooth delivery of Strauss-Kahn, 57, a former economist and convinced 'Social Democrat', has contrasted with the stilted speaking style of Royal, who has little experience of the raw reality of political rough-and-tumble. 'In the debates Strauss-Kahn was very good, open, informing and informed,' said Frederique Dabi, of the IFOP pollsters. 'That made a difference.'

Another blow came late last week with the broadcast of a video, filmed by an activist at a small meeting in January, in which Royal suggests that something should be done about French state school teachers who profit from a light workload to moonlight in the private sector, arguing that they should work a 35-hour week in their schools. Such ideas do not play well with the tens of thousands of teachers who are members of the Socialist Party.

'It's a betrayal, frankly,' Marie Franck, a high school teacher in Marseille, told The Observer. 'She is meant to be on our side.' As the French media hunted the mole who posted the video on the internet, Royal's aides complained of 'low blows'.

But the battle between the candidates is more than a clash of personalities and genders. Each reflects a different strand of left-wing thought and a different type of Socialist voter. According to pollster Dabi, Royal's powerbase lies among women, workers and the young while Strauss-Kahn, who is unashamedly 'Blairist' though he dare not use the word, attracts support from middle-class professionals and even the right. Laurent Fabius is campaigning on a hard left-wing platform, with capitalism, neo-liberal economics and big business as favourite targets. He has strong support among long-term activists who see Royal as a moderate who has sold out.

Though Fabius scores less than 10 per cent in the polls, senior party figures point out 'that the polls probably do not reflect the weight he has in the heart of the party. 'He has established networks and, at the most recent internal party count, scored over 20 per cent,' said Stephane Le Foll, a senior Socialist official.

Even after weeks of gruelling campaigning, there is still much indecision among Socialist party members, particularly the 80,000 who joined as a result of a hugely successful internet campaign earlier this year. 'I just don't know who to choose,' said Najwa Chekini, an administrator who signed up to the party's branch in Paris's 9th arrondissement six months ago, 'My heart says Segolene but my head says Strauss-Kahn.'

Chekini said that a key consideration was the ability of the candidates to take on Nicholas Sarkozy, the maverick Minister of Interior who is widely expected to be the main conservative candidate next year. 'Royal would get eaten alive,' said one activist from the 14th arrondissment. 'I'm voting Strauss-Kahn.'

"It was the other two who wanted these debates because they doubted my capabilities ... But at the end of the process my legitimacy is no longer in question," she told Le Journal du Dimanche newspaper.

"I am the only one who can win against the right. I embody the profound change that people are crying out for. Many see me as the candidate against the powers-that-be .... For the symbolic father of the nation to be a woman -- now that's a revolution," she said Sunday.

A new poll Sunday showed that Royal, who has emerged from nowhere in a year to be France's most talked-about politician, enjoys a clear majority of support -- 58 percent -- among PS sympathisers, with former finance minister Strauss-Kahn on 32 percent and former prime minister Fabius on nine.

However this was a fall of five points compared to two weeks ago, and -- like all polls -- it failed to take into account that the 200,000 who will choose the PS presidential nominee are card-carrying party members and not from the general public.

Both Strauss-Kahn, 57, and Fabius, 60, expressed optimism Sunday that they can together reduce Royal's vote on Thursday to under 50 percent -- thus forcing an unpredictable second round vote a week later between the two leading candidates.

Commentators agreed that Fabius's low showing in polls did not reflect his true standing in the party, where he has extensive networks of influence on the left, while Strauss-Kahn has made steady progress following the debates -- three of which were televised.

Many long-standing members of the party are receptive to criticism that Royal has played on her glamour to build an anti-establishment image that goes down well with the voting public but has little political substance.

However they could be outnumbered by a large intake of new activists -- some 80,000 since the start of the year -- who were drawn by Royal's originality and energy. Many also see her as the best way of ensuring victory over the likely right-wing candidate, ruling party chief Nicolas Sarkozy.

In the latest of a series of collisions with PS orthodoxy, Royal -- who is head of the Poitou-Charentes regional council -- was accused of pandering to the public after she appeared in a pirated video last week berating teachers for not spending long enough in the classroom.

A spokesman described the video's release on the Internet as a "stupid last minute manoeuvre to discredit her".

Previously she has angered the party "elephants" -- as its traditional leaders are known -- by calls for boot-camps for young delinquents, greater freedom for parents to choose schools for their children, and "popular juries" for the public to monitor politicians.

Former socialist prime minister Michel Rocard said over the weekend that she lacks the "authority and respect" which a French president needs on the world stage.

"The next president will be assailed by major international problems the day after his or her inauguration. I do not doubt that Madame Royal is capable of acquiring (authority and respect) but it will take her a year and a half or two," he said.

Rocard said that Fabius -- who urged a "no" vote in last year's referendum on the EU's proposed constitution -- "does not have the respect of the international community", so he urged party members to vote for Strauss-Kahn.

Thursday's vote will take place from 4.00 pm (1500 GMT) to 10.00 pm in some 4,000 regional branches. The results will not be known until Friday morning. The presidential election takes place over two rounds on Sundays April 22 and May 6.