The International Federation of Professional Footballers' (FIFPro) study questioned 576 former top-flight footballers, ice hockey and rugby players from eight countries and found those who had four or five concussions during their career were 1.5 times more likely to have anxiety, depression or sleep disturbance.

The study was carried out by the world players' union's chief medical officer, Dr Vincent Gouttebarge, and sports medicine experts from South Africa's University of Cape Town, Japan's St Marianna University School of Medicine in Kawasaki and Queen Elizabeth Hospital in Glasgow.

"We as football stakeholders - federations, clubs and player unions - need to be alert to the mental health of players, both during and after their careers," said Gouttebarge.

"That means educating players about the dangers of what can be an intense and stressful career and supporting them when they need assistance."

The former players surveyed - all men under 50 and from Finland, France, Ireland, Norway, South Africa, Spain, Sweden and Switzerland - who suffered six concussions were between two and five times more likely to report symptoms of common mental disorders than those who had no concussions, the report discovered. 

The findings suggest during the first 10 years after retirement players are seven to 11 per cent more likely to report these symptoms.

The study claimed, though, there is no suggestion these symptoms of common mental disorders indicate a degree of brain damage.

A separate FIFPro study found that footballers are also more prone to mental health problems after long-term injuries.

"Players who miss more than 28 days through injury are two to seven times more likely to experience symptoms of common mental disorders in the subsequent 12 months than counterparts who did not have any injuries," FIFPro said.