A new football-themed 50p coin designed to ease confusion around the offside law has been written off as "totally out of date" and "confusing" by refereeing experts.

The design, unveiled on Wednesday, is one of 29 coins produced to commemorate the London 2012 Olympic and Paralympic Games, each featuring a different sport.

The football coin – half a million of which are now in circulation – shows a midfielder about to pass to one of two team-mates, with the first player, on the left, marked as offside, and the second, level with the defender, not offside. But the diagram appears to illustrate the offside law as it was until 1995, when it was overhauled by the International FA Board to reduce the number of stoppages in matches.

The revision to the law meant that any player in an offside position when the ball is played is no longer automatically penalised. It states: "It is not an offence in itself to be in an offside position."

Instead, for the past 17 years assistant referees have been told to wait and see whether a player in an offside position becomes involved in active play, either by "interfering with play, or interfering with an opponent, or by gaining an advantage by being in that position". That means that if the midfielder on the coin passes to the striker on his left, but the striker chooses not to play the ball or interfere with an opponent, he is not offside and play continues.

The Royal Mint says the coin was designed "to provoke discussion", but the Referees' Association member Mal Davies, who works with the former head of Premier League referees Keith Hackett on the Observer's long-running You are the Ref feature, said using such old information was "embarrassing".

"The public will assume this has been thoroughly checked, but sadly it's totally out of date," Davies said. "And on parks pitches it will just encourage players to keep pressurising officials to blow the whistle immediately any time a player is in an offside position – and to abuse them when they don't. It's always good to see attempts to explain the Law to a wider audience, and the coin looks good – but unfortunately it takes us back to the last century and just confuses the issue even more."

Launching the football 50p, Susannah Lee, a spokeswoman for the Royal Mint, said the reaction to them had been "phenomenal". "It's an unusual and eye-catching design and has generated significant interest from collectors and sports fans as a result," she said. "The coin depicts a clear representation of the offside rule which I think is useful and easily understood for anyone who happens to find it in their purse or pockets. There are currently half a million of the offside rule 50ps in circulation."

The coins are also being sold by the Royal Mint in presentation packs

The designer of the coin, however, disagrees with Davies. "With all due respect, I reject Mal Davies's interpretation of the coin (rather than his interpretation of the offside law)," said Neil Wolfson. "Nowhere on the coin does it say that the 'offside' player is committing an offence – that is a supposition entirely of Mal's creation.

"The coin simply states that the player is 'offside' – which is true, irrespective of whether or not an 'offside offence' results from this scenario. Furthermore, there are clearly space limitations on the coinface so it was obviously impossible to go into the finer details of the offside rule.

"But for those who don't understand offside, it's not a bad starting point (with further explanation) – and the coin seems to amuse those who do appreciate the offside rule.

"So I simply don't agree with Mal's objection – on the basis that he refers to the 'offside offence', despite the coin having no such reference. I just hope Mal doesn't book me for showing dissent."