The game’s lawmakers have rejected a temporary concussion substitute trial in the Premier League and two other competitions next season, a move which has been criticised by players’ unions, leagues and campaigners.

Here, the PA news agency looks at the key issues.

What’s the background to this?

World players’ union FIFPRO has been calling for a trial of temporary concussion substitutes for the best part of a decade. Last month, FIFPRO and the World Leagues Forum wrote to the International Football Association Board, which governs the laws of the game worldwide, asking for permission to conduct temporary concussion substitute trials in the next seasons of the Premier League, Ligue 1 in France and Major League Soccer (MLS) in the United States.

Why did those groups want to stage this trial?

The temporary substitute protocol gives medics a longer period of time to assess players who suffer a head injury – 10 minutes as opposed to three under the permanent concussion substitute protocol currently being trialled. It also allows medics to conduct that assessment away from the pitch instead of on it. Its backers therefore feel the temporary approach better protects players from brain injury, at a time when studies have found footballers are at greater risk of death from neurodegenerative disease than the general population.

There is an acceptance that even with a 10-minute assessment some concussions will be missed, but that it would provide additional time to identify a greater number of the less obvious injuries than the three-minute assessment.

What did the IFAB decide?

The IFAB could not agree on a consensus at its annual business meeting at Wembley on Wednesday. Football Association chief executive Mark Bullingham, a director of the IFAB, said this meant there would not be support for a trial next season.

The FA had been co-ordinating international efforts to support a trial, and it is understood a trial had majority support among the four UK associations heading into Wednesday’s meeting.

If enough of the FAs supported a trial, why isn’t it happening?

The four UK associations make up four-fifths of the IFAB, with the final constituent being world governing body FIFA. When matters come to the IFAB’s annual general meetings for ratification, FIFA has four votes while the UK associations have one each, with a 75 per cent majority needed for motions to pass. So any request coming before the IFAB effectively requires FIFA support.

Why did FIFA not support it?

FIFA sees the permanent concussion substitution model – when applied correctly – as being safer. Its credo on head injuries is ‘suspect and protect’ –  and its argument is that if you think you need a temporary replacement in order to assess a player, then there is already a suspicion of concussion.

It cites data that up to 25 per cent of players subjected to longer head injury assessments in other sports such as rugby are later found to have concussion, despite being passed fit to play on – so-called ‘false negatives’.

Permanent substitutions, it would argue, come with a zero per cent risk of false negatives because if concussion is suspected after three minutes, the player is withdrawn.

What harm would there be in trialling temporary concussion substitutes?

No adequate answer has been given to this question yet. Perhaps the possibility of ‘false negatives’ from longer assessments is one possible ‘harm’ that opponents of a temporary concussion sub trial can foresee, but the unions and leagues would strongly dispute that.

What is the IFAB doing on concussion then?

Temporary concussion substitution trials will be kept under “active review” but the focus is on making sure the existing permanent concussion sub protocol is correctly applied, and that players are withdrawn when there is a suspicion of concussion. There was an acceptance that there have been a number of instances where the protocol has not been correctly applied and concussed players have been allowed to stay on, but it is understood no specific methods to improve the existing protocol were discussed at the ABM. Permanent concussion substitute trials were given the green light to continue indefinitely by the IFAB.

What was the reaction to the IFAB’s decision?

Dr Adam White, the head of brain health at the Professional Footballers’ Association, said the decision was “extremely disappointing”. Penny Watson, the wife of former England captain Dave Watson who is now living with dementia, described it as “crazy”.

What happens next?

The unions and leagues who made the original request are now considering their options. Lobbying for a change of heart at the IFAB’s annual general meeting in March looks doomed to fail, and in any case that comes too late for MLS to introduce the protocol for the start of its new season on February 25.

Will the MLS be prepared to defy the IFAB and press ahead with a trial? Would FIFA sanction the American league if it did? Might other leagues then follow suit? Watch this space.