BUCHAREST (REUTERS) - Dinamo Bucharest midfielder Patrick Ekeng, who died after collapsing on the pitch during a league match on Friday, was suffering from serious heart problems, Romania's institute of forensic medicine said on Wednesday.

The 26-year-old Cameroon international suffered a suspected heart attack shortly after coming on as a 62nd-minute substitute in a match against Viitorul Constanta. He was pronounced dead two hours later.

Tests confirmed that Ekeng had a serious heart condition, the institute said in a statement.

Romanian doctors said an autopsy conducted on Monday showed that Ekeng had an enlarged heart and said more tests were needed to confirm the cause of death.

Coroner Abdo Salem said he observed an enlarged heart, telling Agerpres national news agency: "He had serious heart problems."

Prosecutors in Bucharest have already opened an investigation into Ekeng's death amid criticism of the treatment the player received.

Romania's interior ministry said on Sunday that the ambulance company which took Ekeng to hospital had faulty equipment and medicine beyond its expiry date in some of its vehicles.

The ministry suspended the licence of the private company for at least 30 days and imposed fines totalling 23,800 lei (S$8,200).