The French prime minister, Manuel Valls, has attacked the opposition UMP party for not advising supporters to vote tactically to keep out the Front National in next Sunday’s deciding round of local elections.

Valls criticised the decision by the UMP – led by Nicolas Sarkozy – as a “moral and political fault”.

“When you have the choice between a republican candidate or the Front National, you don’t hesitate. In any case, the left doesn’t hesitate,” Valls told RTL radio.

“The ni-ni vote (neither nor) is not the solution to the UMP’s problems,” Valls added.

His comments came as the final results in the first round of voting showed the ruling Parti Socialiste trailing on 21.6% of the vote, behind the Front National on 25.2% and the UMP and centre parties on 29.4%.

Early estimates had suggested the PS had narrowly beaten the FN into second place, but the eventual count showed it had dropped to third place.

Government supporters argued that support for other breakaway left groups – including the far left Front de Gauche and the Europe Ecology Green party – brought the leftwing total vote up to 28.7%.

However, the only real consolation for François Hollande’s beleaguered administration was that the FN had failed to do as well as polls had predicted.

The PS, which until Sunday controlled 61 of France’s 101 departments, can now expect to win around 20 in next Sunday’s second round vote, and the FN four or five.

The PS suffered the symbolic loss of the Seine-et-Marne department outside Paris that it has held for a decade, and suffered humiliation in the gritty working-class banlieue of Seine-Saint-Denis, with jut 7.7% support, compared with 20.2% for FN, and 9.9% for the UMP. The Union de la Gauche, an alliance of leftwing parties polled 20.5%.

Sarkozy has sparked anger among the left for failing to advise supporters to form a block against the FN in areas where the UMP stands little or no chance of winning a seat. France’s Socialist government has advised its voters to do whatever necessary to keep out the FN.

Former government minister and Socialist rebel Benoît Hamon voiced the ire of many on the left saying it was a shame the UMP “is again playing the ni-ni card”.

“That the UMP considers that the Socialist party is as toxic as the FN concerns me,” he tweeted.

Socialist party officials have reminded voters that when Marine Le Pen’s father, Jean-Marie, caused an electoral earthquake by winning his way into the second round of the presidential election in 2002, it advised supporters to vote for the right’s candidate Jacques Chirac.