(Corrects attribution in fourth paragraph to source close to investigation from regulator CNBV)
* Net profit drops 18.7 pct, core profit slips 5.4 pct
* Mexico motorway business weighs on results
* Peers FCC and Abertis see profit growth in Q1
By Julien Toyer
MADRID, May 13 (Reuters) - Spanish infrastructure group OHL posted an 18.7 percent drop in first-quarter net profit on Wednesday, hit by a lower contribution from its motorway business in Mexico where the firm is investigating allegations about overcharging the government.
Shares in OHL Mexico have fallen 20 percent since recordings appeared on YouTube in which executives are heard discussing overcharging for public works.
The company has launched an investigation into the matter, but says it has not acted illegally and that the recordings were edited to distort their content.
Separately, a source close to Mexico's securities regulator CNBV on Wednesday said the agency had launched a review of OHL Mexico's disclosures to the country's bourse due to the disclosure of the recordings.
The governor of the State of Mexico has also ordered a state audit to ensure OHL Mexico abides by the terms of its highway contract.
The Mexican business accounts for 72 percent of OHL's core profit and its performance is largely dependent on the inflation rate in the country, which came in at 0.51 percent in January-March versus 1.43 percent a year ago.
This sent group net profit down to 49.1 million euros ($55.3 million) from 60.4 million euros last year and weighed on earnings before interest, taxes, depreciation and amortization (EBITDA), which slipped 5.4 percent to 263.9 million euros.
The negative impact more than offset growth in the engineering and building division, where EBITDA was up 36 percent, as well as in other Latin American and Spanish motorways, where traffic increased as Spaniards use their car more amid falling gasoline prices and an economic recovery.
Spanish peer Abertis also saw 6 percent traffic growth at its domestic highways, boding well for the group's strategy to focus on motorways after it sold its car park and airports businesses and listed part of its telecoms unit.
Traffic was also up in France and Chile and slightly down in Brazil, Abertis said on Wednesday.
Meanwhile, results from another competitor, FCC, showed it had emerged from a deep restructuring which saw Mexican billionaire Carlos Slim becoming the firm's largest investor, although continued weakness in its construction and cement arms indicated turning around the business would take time.
FCC said on Wednesday it returned to net profit after two and a half years of quarterly losses, with net profit reaching 6.2 million euros.
$1 = 0.8887 euros Additional reporting by Alexandra Alper in Mexico City; Editing by Mark Potter and Lisa Shumaker