A former youth player for Manchester United today won more than £4.3m damages at the London high court for his injuries from a tackle that ended his career.

The amount awarded to Ben Collett - described by the Manchester United manager, Sir Alex Ferguson, as an "outstanding" player – is the largest ever compensation payout to a professional sportsperson in the UK. But the sum will go up when future loss of pension and interest on lost earnings are determined at a hearing set for October.

Mrs Justice Swift, sitting in London, today announced that when all of the figures were decided, the final sum to be awarded was not likely to be less than £4.5m.

At a recent hearing in Manchester, Ferguson said Collett, now 23, had an "outstanding chance" of becoming a full-time professional if he had not been injured. Collett joined United's youth academy aged nine and signed youth contracts, becoming part of the club's winning FA youth cup team in 2003. But aged 18, and in his first game for the reserves in May 2003, he had his right leg broken in two places in a tackle by Middlesbrough's Gary Smith.

Smith and the club admitted liability for the "negligent" tackle and the judge's task was to decide on the level of compensation Collett should receive from Middlesbrough's insurers.

Mrs Justice Swift said had it not been for the injury, Collett would have been offered a three-year professional contract with Manchester United in July 2003.

In a statement read out at court, Jan Levinson, a partner at law firm Beachcroft, which represented Collett, said the size of the award reflected "his talent and potential prior to the tackle as one of the brightest young footballers in the country".

Levison added: "The judgment announced today is the highest award ever given to a professional sportsman or sportswoman. Thankfully, injuries of this severity are not a common occurrence on the football or any sporting pitch, so I do not see this successful claim opening the floodgates to litigious action by sportsmen."