The No campaign gained support after anti-government strikes

The poll, in the Paris newspaper Le Parisien, indicates that 51% of voters will vote No to the constitution in the 29 May referendum.

Leader of the ruling UMP party Nicolas Sarkozy said a No vote in France would either paralyse EU or isolate France.

Spain was the first country to back the text by referendum but turnout was low.

The treaty must be ratified by all 25 member states, either by referendum or parliamentary vote and several countries are planning to let the public decide.

PM 'preoccupied'

In France, recent polls had shown that 63% of voters would support the constitution.

But the CSA poll of 802 people published in Le Parisien shows a drop in that support.

In the poll, 51% said they would oppose the constitution if a vote were held today, with 49% in favour. The figure has a margin of error of 3%.

The CSA survey said support for the No campaign rose from 31% six months ago to 37% in mid-February, before increasing as France was gripped by anti-government strikes and demonstrations.

French Prime Minister Jean-Pierre Raffarin said he was "preoccupied" by the latest figures, but not worried.

"If the result were known in advance, people would not feel responsible. With opinion split 50-50, the French people will be personally responsible for the choice they make," he told young members of the European People's Party.