French Prime Minister Manuel Valls today blasted a planned EU-US trade treaty, saying the ambitious deal was against "EU interests."

"No free trade agreement should be concluded if it does not respect EU interests. Europe should be firm," Valls told members of the governing Socialist Party, adding "France will be vigilant about this."

"I can tell you frankly, there cannot be a transatlantic treaty agreement. This agreement is not on track," Valls said.

The Transatlantic Trade and Investment Partnership (TTIP) aims to topple regulatory and tariff barriers and establish the largest free-trade and investment area in the world.

The United States and the European Union (EU) have been negotiating the deal for the last three years.

The two sides have been pushing to resolve remaining issues by year's end, coinciding with the end of Barack Obama's presidency. The next round of negotiations is expected in July.

But the project has been facing mounting opposition in parts of Europe, especially in France and Germany, where critics say the talks have been conducted in secret and fear a far-reaching impact on agriculture and the environment.

The TTIP agreement "would impose a viewpoint which would not only be a breeding ground for populism, but also quite simply be a viewpoint that would be bad for our economy," Valls said.

He pointed in particularly to the "dramatic" consequences of ending milk quotas. The dairy industry is one of France's prime sectors.

Valls was speaking in the wake of Thursday's "Brexit" referendum which will lead to Britain - a firm supporter of free trade - leaving the 28-nation EU.