MEXICO CITY, May 24 (Reuters) - Pachuca coach Jose Luis Trejo has resigned 48 hours after his team won the Mexican championship and joined rivals UANL, the club said on their official website.Pachuca, who beat San Luis over two legs to win their fourth title since 1999, angrily accused the Monterrey-outfit nicknamed the Tigers of unethically poaching the coach. 'The coach....announced his decision to leave Pachuca and join UANL as their financial offer was attractive,' said Pachuca in a statement on Tuesday. 'It's very sad to see a great institution such as the Tigers, who have one of the best sets of fans in the country, behaving in this unethical manner.' Pachuca's sporting director Andres Fassi said in a radio interview: 'Trejo told us that UANL's offer was very good, that he didn't want to miss out on it and.....in spite of all our arguments, he had already made the decision.' Pachuca, nicknamed the Beavers, are Mexico's oldest club and were founded by a group of miners from the English county of Cornwall in 1901. After spending many years in the second division, Pachuca returned to the top flight in 1996 and have flourished under the ownership of local businessman Jesus Martinez. Trejo will replace Brazilian Ricardo Ferreti who resigned earlier this month. On Tuesday Ferreti took over at Mexico City-based UNAM where he will became the third coach since former Mexico and Real Madrid striker Hugo Sanchez was fired last October. Ferreti said his priority was to save the Pumas from relegation, which is decided over three years and where consistently poor results over the last 18 months have left the team close to the danger zone. Sanchez, who won two successive titles with the Pumas in 2004, was first replaced by Miguel Espana and then by Guillermo Vazquez, who will work as Ferreti's assistant. Two championships - the Apertura and Clausura - are played in the Mexican season.