With the French left in the doldrums, opinion polls suggest Fillon will face Le Pen in the second-round run-off of the presidential elections next May.
Le Pen is adamant that the same anti-establishment anger which saw Britain vote to leave the EU and Americans elect Donald Trump as president could sweep her to power, although polls suggest that is unlikely.
At well over four million, turnout in yesterday's primary was higher than in the first round of voting last week, when Fillon knocked former President Nicolas Sarkozy out of the race with a late surge in support.
A racing car enthusiast who lives with his Anglo-Welsh wife Penelope in a chateau in the Loire valley, Fillon is a social conservative who has positioned himself as a defender of traditional family values and France's Catholic roots and an opponent of "multiculturalism".
He has also taken a hard line on Islam, saying the religion must change in the wake of terror attacks that have left 230 dead in the past two years.
"The Islamic religion (must) accept what all the others have accepted in the past ... that radicalism and provocation have no place here," he said at his final Paris rally last weekend.
On the economic front, he has promised to break with France's statist tradition and to roll out an ambitious free-market programme that will undoubtedly set him on collision course with France's unions.
His pledge to slash half a million state sector jobs, jettison the 35-hour week, and pay 'fonctionnaires' 37 hours for working 39 were branded "brutal" and unworkable by Juppe.
Fillon backs the idea of a "Europe of nations" and has voiced scepticism over the role of the European commission and EU Parliament.
Regarding Britain, he has called for a "good neighbourly deal" over Brexit, but recently made it clear that if the UK refused the free movement of EU citizens, there is no "reason to leave them the European financial passport and the eurozone must take back clearing its currency".
He also wants to return British border controls, which currently operate in Calais, to the UK.
There are concerns about Fillon's support for a US alliance with Vladimir Putin, the Russian President, over Syria. Britain is strongly opposed to such an alliance amid concerns that it will bolster the Bashar al-Assad regime.
Juppe, 71, had proposed similar, if slightly less radical, economic reforms. The mild-mannered moderate had gunned for the centre and left-wing vote by expressing confidence that France's ethnic and religious diversity could forge a "happy identity".
As his early lead waned, he complained bitterly of an online smear campaign by the far-right to depict him as soft on Islamic fundamentalism.
Juppe suffered another blow when television viewers found the harder-line Fillon more convincing in a head-to-head debate last week.
Pollsters say the winner of the centre-right primary will likely beat Le Pen and Juppe centred his campaign on being the best consensus candidate to do so. Fillon's camp argued that he will pose a tougher challenge for Le Pen, forcing her to tack left.
Attention will now turn to the French left, with a bitter contest expected between Francois Hollande, the most unpopular president of modern times, and his Prime Minister, Manuel Valls.
