French President Nicolas Sarkozy inched up to just half a percentage point behind his Socialist rival Francois Hollande in a daily opinion poll on Monday, the closest gap of any survey so far for the April 22 first-round vote.

The Ifop/Fiducial survey gave conservative Sarkozy 28 percent, up one point from Sunday, to Hollande's 28.5 percent, down half a point, reducing Hollande's lead from 3.5 points at the start of March.

The poll still showed Hollande a full 10 points in the lead for a May 6 runoff, however, with 55 percent to Sarkozy's 45 percent.

The election is shaping up as a clear two-horse race between Sarkozy, who promises tighter immigration controls, structural reforms and policy referendums, and Hollande, who is running on a tax-and-spend programme while also promising to cut the budget deficit.

The poll was the first since Sarkozy gave his campaign a new twist on Sunday by threatening to erect barriers to trade and immigration unless the European Union takes tougher stands.

Seeking to breathe new life into his campaign, Sarkozy told his biggest rally yet that Europe should have a law, modelled on the Buy American Act, requiring governments to buy European-made products.

He also threatened to pull France out of the European Union's Schengen open-borders zone unless progress is made over the year on controlling immigration flows.

Hollande said Sarkozy's move was a sign that Sarkozy was running out of inspiration and shrugged off the latest poll showing his first-round lead dwindling.

I am not going to let myself be impressed by a poll, Hollande said on France 3 TV.

Sarkozy is seeking a second term despite being one of the least popular presidents in modern France, with many voters frustrated with his poor economic record and tired of his showy style.

Separately, an Opinionway survey showed Sarkozy's approval rating slipped one point to 31 percent while his disapproval rating dropped two points to 62 percent.

It found that 50 percent of respondents thought Hollande was running the stronger campaign and 38 percent rated firebrand leftist Jean-Luc Melenchon's most highly. Only 27 percent thought Sarkozy's campaign was best.