Mr Hearn was a 26-year-old Oxford Blue with seven England caps when he broke his neck during a match between London and Midland Counties and the All Blacks in October 1967.
Giving evidence from his wheelchair on behalf of Ben Smoldon, the 21- year-old hooker who was left tetraplegic when a scrum collapsed during an October 1991 under-19 Colts game between Sutton Coldfield and Burton on Trent, Mr Hearn criticised the standard of control exercised by match referee Michael Nolan. He said that if, as alleged, it was correct that there were 25 scrum collapses before Mr Smoldon's accident, Mr Nolan had fallen far short of the standard of the Staffordshire Society of Rugby Football Union Referees.
"I think it's a quite extraordinary number of collapsed scrums and the fact he hadn't been authoritative enough to prevent the number of scrums would suggest to me that he isn't a referee of the highest competence," he said. "And particularly a referee refereeing at that level, where he has an excessive duty of care for the protection of those young people."
Mr Smoldon of Sutton Coldfield, West Midlands, is suing Mr Nolan and Thomas Whitworth, a member of the opposing team, who both deny liability. He claims the match was not played according to the laws of the game or in a sporting spirit.
Mr Hearn, who was in charge of rugby at Haileybury College public school for 15 years, said a referee's job was to be a calming influence and take the steam out of young enthusiastic players.
It was a matter of huge importance that he should "act before the storm". "The potential for danger is greater at under-19 level which is exactly why the laws were changed to reduce the risk element in the game.
He added: "Rugby football is not a game for young people to sort each other out . . . If scrummages continue to be unstable and, in fact, dangerous, he [the referee] has no option but to stop the game."
Earlier, a team-mate, Robert Elias, said he started the game as hooker, but swapped with Mr Smoldon, who was on the wing, after 15 minutes because his neck was sore from the scrums. "I couldn't take too much more pressure on my neck," he told Mr Justice Curtis. "I then had an outside view of what had been happening to me. The same kind of occurrences were happening - the scrums were being engaged with great force and had to be reformed."
The case continues today.Reuse content