The head of a major French trade union said Monday that he would join protests against President Emmanuel Macron after previously opting out, signalling new resistance to the centrist leader's reform agenda.

Jean-Claude Mailly, secretary general of the hard-left Force Ouvriere (FO) union, had surprised many observers by choosing not to take part in demonstrations against changes to labour law pushed through by Macron last month.

After months of insisting that unions needed to discuss and negotiate with the new government, on Monday Mailly bowed to pressure from rank-and-file members by saying the FO would join a "day of action". 

"We'll be in contact with the other confederations in the days ahead," Mailly told reporters while criticising a "number of unacceptable measures" unveiled by Macron's ministers recently.

Divisions in the labour movement were seen as one of the reasons the 39-year-old president had so far not faced major street protests, with two separate days of demonstrations called by the Communist-backed CGT failing to generate momentum.

France's third major union, the more moderate CFDT, has also signalled it will harden its tone against Macron's pro-business agenda, which has brought major changes to employment laws governing hiring and firing.

The next priorities for Macron's government include an overhaul of the unemployment benefits system, which is likely to be highly controversial, and then pensions as part of a promised "transformation" of the social system. 

The government's first annual budget has also sparked a backlash from opponents on the left because of major tax cuts for the wealthy, which Macron sees as essential to spur investment.

Anger is also growing at the local adminstration level in France, where mayors are concerned about planned cuts to their funding as part of efforts to trim France's budget deficit.

"Macron in a trap," headlined the Journal du Dimanche newspaper at the weekend, predicting that the government would face rising resistance as it pushed forward with its ambitious plans.