PARIS • The four-candidate battle to reach the run-off in France's presidential election is putting pollsters to the test as never before.

With just a few days to go before Sunday's first round of voting, every poll for the past month has shown independent Emmanuel Macron and the National Front's Ms Marine Le Pen in the lead.

Mr Macron would then easily win the May 7 run-off, polls show. Yet both front runners have been slipping in the past two weeks, and Republican Francois Fillon and Communist-backed Jean-Luc Melenchon are now within striking distance. It is a challenge for French pollsters, which have a near-perfect record in forecasting the vote share for the top five finishers in the first rounds in 2007 and 2012 and the subsequent run-offs.

Until recently, the expectation was that France would not have an electoral shock like Britain did with Brexit and the US with the election of Mr Donald Trump.

"This situation is totally unprecedented," said Mr Emmanuel Riviere, managing director of Kantar Public France. "The fact that there are four potential finalists makes the situation very complex."

French political pollsters are aided by heavier reliance on Internet polling than in the US and Britain. And French elections are simple - one person, one vote. The two- round system means a straight face-off between the top two candidates in the run-off.

The difference in this year's first round is that the top four candidates are within a range of fewer than 4 percentage points. Given margins of error that are typically between 2.5 points and 3 points, the race is tighter than it might initially appear. On top of that, as many as 40 per cent of voters have yet to decide, according to estimates by polling firms.

As of Tuesday, Mr Macron was running at 23 per cent and Ms Le Pen at 22.3 per cent, according to the Bloomberg composite of French polling. Mr Fillon and Mr Melenchon were both at 19.5 per cent. In the second round, Mr Macron would defeat Ms Le Pen by 64 per cent to 36 per cent, according to Opinionway, which also says Mr Fillon would defeat Ms Le Pen 58 per cent to 42 per cent.

BLOOMBERG