Developed by New Zealand kit firm Canterbury, the 'IonX' shirts are made from a fabric that contains a negatively charged electromagnetic field.

This means the process of ionisation, which produces electrically charged atoms long understood by former Eastern Bloc scientists to improve sporting performance, can occur in the body during training and even competitive matches.

It is claimed to increase blood flow and oxygen levels, leading to improved performance.

Portsmouth seem to think it has an effect.

Paul Bell, the club's commercial director, said: 'It's something we believe is delivering a tangible, positive effect on our players.'

Portsmouth are so keen they have done a deal preventing other Premier League sides from using IonX for 'the immediate future' — thought to be several seasons.