PARIS, Feb 4 (AFP) - Jitters over French ratification of the EU's constitutional treaty were increasingly apparent Friday after the country's largest trade union defied its leadership to recommend a ‘no’ vote in the national referendum later this year.

The General Labour Confederation (CGT), which was until recently linked to the Communist Party (PC), voted overwhelmingly in its 120-member national committee to reject the constitution on the grounds that it will entrench free-market economics at the expense of "social priorities."

The decision, which was against the wishes of CGT secretary-general Bernard Thibault, came amid signs that a significant part of the French left is prepared to ignore the advice of its hierarchy and vote against the text when it is put to the people probably in June.

Despite an internal ballot in favour of the treaty late last year, the main opposition Socialist Party (PS) remains deeply divided - with 56 deputies out of 140 defying party leader Francois Hollande Tuesday this week to abstain in a National Assembly debate on the issue.

With a recent wave of street protests indicating high levels of popular discontent in France, the centre-right government of President Jacques Chirac fears a tide of feeling against the constitution which will overturn the ‘yes’ vote currently predicted in the opinion polls.

In the 1992 referendum on the Maastricht treaty that paved the way to the EU's single currency, a strong early lead by supporters was whittled away to a razor-thin majority as voters used the opportunity to express unhappiness with then-president Francois Mitterrand.

And the first round of France's 2002 presidential election - in which nearly one in three voters chose candidates of the far-right or far-left - remains a warning of how easily the public can swing out of the political mainstream.

Many left-wing voters who reluctantly backed Chirac in round two of that election in order to defeat far-right leader Jean-Marie Le Pen feel it will be a humiliation too far to hand him another victory in the referendum on the constitution.

"There is a real danger that popular anger will lead a majority to reject the constitution in a sheer protest vote," the left-wing Liberation newspaper said in an editorial Friday.

Support for the constitutional treaty comes from Chirac's ruling Union for a Popular Movement (UMP) party, its ally the Union for French Democracy (UDF) and the opposition Greens - assuring a clear majority in parliament.

The PS tried to resolve its internal differences in a membership ballot in December and the party leadership also supports the treaty. However the 40 percent of members who rejected the text then continue to speak out behind former prime minister Laurent Fabius, who remains Hollande's deputy.

Of parliamentary parties only the Communists formally oppose the treaty. Describing the CGT vote as "tremendous," party secretary Marie-George Buffet called on "workers to mobilise to ensure the failure of liberal policies and to use the referendum to demand different policies for France and Europe."

Opposition to the constitution is also argued by Le Pen's National Front as well as sovereignty advocates such as Philippe de Villiers on the right and former interior minister Jean-Pierre Chevenement on the left.

A row this week over the EU's so-called Bolkestein directive which would liberalise service industries across the 25 member states has exposed Chirac's vulnerability to attacks from the left that the constitution is a charter for big business.

Fearful that compliance will hand further ammunition to the constitution's opponents, Chirac and Prime Minister Jean-Pierre Raffarin both said the directive is unacceptable in its current form.

The EU constitution, which is designed to avoid decision-making gridlock in the expanding bloc, requires the formal approval of all 25 EU states - and in theory a rejection by just one country could scupper it.

This week Slovenia became the third country to ratify the text, after Hungary and Lithuania.