Founded by nutritionist Zef Eisenberg who has now exited the company, Maximuscle will join brands such as Lucozade and Ribena under GSK ownership.

Eisenberg sold a majority stake to Darwin Private Equity for £75 million in 2007 but stayed on as a significant shareholder and continued his role as president and ‘industry ambassador’.

The acquisition comes on the back of a deal to distribute Maximuscle products in high street retailers such as Argos, Tesco and Waitrose.

Eisenberg comments: ‘We had no intention of selling the business at this time, but GSK approached us quietly with tomorrows price today.

‘We are delighted to have found a buyer that has a proven track record in developing brands and taking them to the next stage.’

The sports nutrition brand, which is endorsed by London Wasps Rugby and Fulham Football Club, launched two new products in 2010, with Maxitone specifically targeting female gym users.

A GSK spokesman comments: ‘We are pleased to acquire this highly regarded brand and benefit from the synergies of having Maximuscle within the GSK camp.’

Mr Eisenberg founded the company in 1995 and has been asked by GSK to stay on as a consultant with no restrictions to his other ventures.