Mexican presidential front-runner Enrique Pena Nieto has extended his big lead over ruling party candidate Josefina Vazquez Mota with barely three months to go until the election, an opinion poll showed on Monday.

The voter survey by polling firm Buendia & Laredo for newspaper El Universal showed backing for Pena Nieto, a member of the opposition Institutional Revolutionary Party, or PRI, at 42.5 percent, up from about 39 percent in a poll in February.

By contrast, support for Vazquez Mota, who is running for President Felipe Calderon's conservative National Action Party, or PAN, fell to 23.7 percent from some 25 percent.

Leftist Andres Manuel Lopez Obrador, who narrowly lost the 2006 presidential contest to Calderon, was at 16.9 percent, compared with just under 16 percent in the last survey.

The latest poll comes just a few days before the official start of campaigning on Friday. The presidential election takes place on July 1.

It was conducted between March 14 and 18 and surveyed 1,000 eligible voters. It had a margin of error of 3.5 percentage points, the newspaper said.

A separate poll from Excelsior daily, conducted between March 17 and 20 among 1,200 eligible voters, showed Pena Nieto maintaining his lead at 47 percent, exactly the same support than in the previous survey from pollster BGC three weeks ago.

Vazquez Mota gained one percentage point to 30 percent in the BGC poll while Lopez Obrador lost a point to 22 percent, according to Excelsior. BGC said its survey had a margin of error of 2.9 percentage points.

The PRI, which ruled Mexico for seven decades until 2000, is hoping the telegenic Pena Nieto can return the party to office, 12 years after it was ousted by the PAN.

The PAN has struggled to create enough jobs for the country's growing population, and has been mired in a bloody struggle with drug cartels, whose turf wars and clashes with security forces have claimed 50,000 lives in five years.

Calderon, who is not allowed to run for a second six-year term in office, staked his reputation on bringing the drug gangs to heel, but the bloodshed has hurt his party's chances of retaining the presidency.

Adding to its problems, the PAN has fallen prey to infighting and defections in the past few weeks following disputes over the party's selection process for candidates in congressional elections also being held on July 1.

(Reporting by Dave Graham; Editing by Peter Cooney and Bill Trott)