"We have to act differently. We have to speak differently", said Valls on Sunday at a meeting with social democrats in Italy.
"We know what will be the terrible price of failure" said French PM as the crisis in France and within the Socialist Party led to an emergency reshuffle of the government last month, but not to the end of turmoil for the ruling Socialists.
An opinion poll on Friday showed that Front National leader Marine Le Pen would beat President François Hollande in next 2017 presidential elections.
Valls, whose reformist agenda caused a recent split in the government, warned that the left across Europe has to change its approach.
"In France, the extreme right of Marine Le Pen is at the gates of power", he said, "and as a man of the left, I will never be able to resign myself to that (...) it will be a fatal blow to Europe".
Valls is the most popular Socialist politician in France but on 16 September, his government will face a vote of confidence in parliament which could - if left-wingers side with the right - lead to a dissolution of parliament.