Any member of the Professional Footballers' Association aged 50 or over is eligible to take part, with all the assessments conducted via the telephone or online, but so far the study leaders have only got around 200 of the 300 participants they would ideally like.

"It's going OK, but maybe not as fast as we'd hoped, so it was good to have support from Gareth Southgate," Professor Neil Pearce, from the London School of Hygiene and Tropical Medicine, told the PA news agency.

"There are a number of reasons for that. One of them is Covid and the various lockdowns. We're going to get to the numbers we require, but it's just taking a bit longer than we expected. Having Gareth's support is a big help."

The study forms part of the Football Association and PFA's efforts to examine what causes the link between an increased risk of neurodegenerative disorders and a career in the professional game, which was established by the FIELD study in 2019.

A similar study among former professional rugby players, called BRAIN, involved physical assessments such as a grip strength test and a 'chair rise' test to measure lower body mobility.

However, the coronavirus pandemic means those assessments will not be a feature of the HEADING study as was originally intended, and all information will be gathered without face-to-face contact.

"(The tests) are fairly standard things, like a series of pictures where there's a name, there's a face and a job," Professor Pearce said.

"Then 30 minutes later you might be shown the face and be asked to remember the name and the job.

"We're not looking for major problems – what we are looking for is minor differences in cognitive function. I know myself as I get older I don't remember things quite so well and I slow down a bit and that's quite normal with age, but we're trying to see at any given age if there's a difference in people who have had a lot of concussions, for example, and people who have headed the ball a lot."

Professor Pearce confirmed participants would also be asked to recall elements of a short story immediately, and then again at a later point.

He said the study team will be looking for "subtle differences" in how individuals perform.

"It's generally accepted that if you are going to have problems like Alzheimer's later in life, it will begin to show up at an earlier age, that you might score a little bit less on some of these cognitive function tests," he said.

"It doesn't mean you're destined to get Alzheimer's, it just means you're slightly higher risk. The idea is that we can see if there's a problem in the 50-60-70 age group that might predict serious problems later in life."

Professor Pearce said individuals could come forward if they believed they were eligible but had not yet been contacted – the researchers would just need to check that the ex-player was on the PFA's database.

The study will be the first that asks participants to provide information about heading. Performance on the cognitive function tests can be compared to age-matched members of the general population.

"We're not just asking people how many times they head the ball," Professor Pearce said.

"We might ask that, but we ask for a lot of other information about their playing career, what leagues they were in, what position they played, about heading in training and so on. And then we put the data together.

"So it's not like someone says, 'I headed the ball so many times' and that's the value we use – we would use the value for everyone else who has the same sort of characteristics and the same sort of playing career. We believe that we can get reasonable estimates, at least (grouping people) into high, medium and low frequency of how much they head the ball."