BOLOGNA, Italy (AFP) - The far-right National Front is at the "gates of power" in France, Prime Minister Manuel Valls said Sunday in a stark assessment of the crisis embroiling the country's centre-left government.

Valls's warning follows weeks of turmoil for the ruling Socialists which culminated in an opinion poll Friday showing that FN leader Marine Le Pen would beat President Francois Hollande in presidential elections in 2017.

The French premier, whose reformist agenda caused a split in the Socialist Party which triggered the current crisis, made his comments in a speech to fellow social democrats from across Europe at the Festa de l'Unita in Bologna, an annual talk-fest in the one-time stronghold of Italy's now defunct Communist Party.

In what will be interpreted as a warning shot across the bows of left-wing rebels inside his own party, Valls, said the left across Europe had to change its approach.

"We have to act differently. We have to speak differently," he said. "In order to be listened to and to be heard.

"We know what will be the terrible price of failure. "In France, the extreme right of Marine Le Pen is at the gates of power," Valls said. "And I, as a man of the left, will never be able to resign myself to that because it will be the weakest who will be the first to suffer. And it will also be a terrible, perhaps fatal, blow to Europe."

The crisis within the Socialist Party which led to an emergency reshuffle of the government last month has been followed by a string of grim updates on the state of France's flatlining economy.

Hollande's woes have been dramatically compounded by the contents of a memoir published by his spurned former partner Valerie Trierweiler, which depicts him as a self-serving opportunist with no principles.