Traumatic brain injury expert Dr Willie Stewart believes it was "only a matter of time" before former rugby players with concussion-related issues considered legal action.

A group of former internationals, including England World Cup winner Steve Thompson, are planning legal action for negligence against the rugby authorities over brain injuries they have suffered.

The 'test group' for a potentially much larger action also includes another former England player, Michael Lipman, and ex-Wales international Alix Popham, according to Rylands Law.

The planned action is against World Rugby, the Rugby Football Union and the Welsh Rugby Union, for "failure to protect (the claimants) from the risks caused by concussions".

Dr Stewart led the FIELD study research, commissioned by the England's Football Association and Professional Footballers’ Association, which revealed last year that footballers were three-and-a-half times more likely to die of neurodegenerative disease than age-matched members of the general population.

"As we’ve learnt more about concussion and head injuries in sport, and the consequences of it, then I think it was only a matter of time before former players experiencing concussion problems sought some sort of legal status as to what is going on," he told the PA news agency.

"Prevention is better than any cure. In terms of rugby, although they’ve made great strides in attempting to identify brain injuries on the park, the number of brain injuries in rugby is still phenomenally high.

"In terms of the number of brain injuries and concussions identified, it is only a fraction of the number that are actually occurring.

"And then there is the problem of head impacts themselves, never mind concussions. Cumulative exposure to head impacts, we believe, is just as much a risk as a handful of concussions."

Thompson, 42, cannot remember winning the World Cup with England in 2003 and says he wished he had never turned professional, while 40-year-old Lipman described himself as "a walking time bomb".

Thompson (centre) says he was concussed regularly without realising

Dr Stewart, a consultant neuropathologist who is also a clinical associate professor at the University of Glasgow, added: "Rugby should be thinking about immediate measures to try and deal with this.

"Whether it is rugby, whether it is football, any of these sports where the head is impacted frequently and regularly, have to be asking questions about whether it is sustainable to carry on like that.

"Or, are we going to see this scenario of problems over the next decade, or even longer. The reason that things are so slow to change is because the exposure and the outcome are 30-40 years apart.

"Players being concussed and injured nowadays, it will be 40 years before we are starting to talk about them and the damage that has been done.

"Sport is not necessarily acting quickly because they don’t see the immediate effects of this, except now with players coming forward with lingering concussion issues, chronic headaches etc, they may be forced to act quicker."

World Rugby told BBC Sport: "While not commenting on speculation, World Rugby takes player safety very seriously and implements injury-prevention strategies based on the latest available knowledge, research and evidence."

The WRU told PA that, as it has had "no direct communication on the subject", it would "just like to support the World Rugby comment", while the RFU has also been contacted for comment.