(Updates with result of election, comment from PAN leader, reaction)
MEXICO CITY, May 18 (Reuters) - The leader of Mexico's biggest opposition party comfortably won re-election on Sunday, boosting President Enrique Pena Nieto's efforts to enact measures to open up the oil industry and increase competition in the telecoms sector.
The center-right National Action Party (PAN), has been badly divided by an internal power struggle centering on how far the party should cooperate with Pena Nieto's Institutional Revolutionary Party (PRI), which lacks a majority in Congress.
In the election, PAN leader Gustavo Madero, who has worked with Pena Nieto to pass reforms, defeated his younger rival Ernesto Cordero, a former finance minister keen to push the party in a direction more critical of the government.
Pena Nieto has relied on PAN votes to help push through laws, including a bill in December to end Mexico's 75-year-old oil and gas monopoly that is the central plank of his efforts to end years of underperformance by Latin America's second biggest economy.
The two parties also worked together on a major overhaul of the telecoms and broadcasting industries dominated by multi-billionaire Carlos Slim and broadcaster Televisa respectively.
Both reforms still need rules and regulations to be set out in so-called secondary legislation, but disputes in Congress, partly due to PAN infighting, have delayed the approval process.
After preliminary results showed Madero won nearly 57 percent of votes cast by PAN members, the party leader quickly turned to the legislation pending in Congress.
"What we must do from today is approve the best secondary laws possible to carry on making Mexico more modern and more democratic," Madero told cheering supporters in Mexico City.
The centrist PRI had hoped to pass the energy and telecommunications secondary laws by the end of April at the latest, and now has its sights on June.
Cordero, who took just over 43 percent of the vote, said he would respect the result and called on the PAN to return to being a "responsible and firm" opposition to the PRI.
During a bad-tempered election campaign, Cordero frequently accused Madero of selling the party out to Pena Nieto. However, Cordero supported the energy reform and other major bills.
Madero argues he is restoring the PAN's fortunes by forcing Pena Nieto to adopt his party's vision of economic reform. That strategy yielded fruit with the energy law, which ended up much more liberal than the one the president first proposed.
Federico Berrueto, director general of polling firm GCE, said the outcome of the vote looked like the PRI and the PAN would be able to return to the spirit of negotiation that existed before the PAN got distracted by the election.
(Additional reporting by Noe Torres; Editing by Sophie Hares and Robert Birsel)