Johnson has fallen out of favour at Chelsea, for whom he has made 68 appearances since joining from West Ham for £6m in 2003. The 21-year-old had also spoken to Dutch club Feyenoord but decided to join the south-coast club.

Pompey boss Harry Redknapp told the club’s website, pompeyfc.co.uk: “It is no secret that I have long admired Glen.

“He is a top defender and will continue to get better, so this is a real top quality capture for us.

“The lad has a great future. He is already an outstanding defender and the kind of player most Premier League sides would give their right arm for.”

Johnson becomes Pompey’s first major signing since narrowly escaping relegation from the Premiership.

He said: “Moving to Portsmouth is a great opportunity for me to play regular Premier League football again and hopefully force myself back into the England set-up ahead of the next European Championships.

“The club have big ambitions and I’m looking forward to playing a part in them.”

Adams, 39, will officially take up his role on Saturday.

Redknapp said: “Tony is a guy I have a lot of respect for and feel he can do a great job for us.

“He knows the game inside out and I’m really looking forward to him joining Pompey and to starting to work alongside me.”

Following his spell at the helm at Wycombe, Adams focused on gaining experience and coaching qualifications in England and Europe.

In May he revealed he was on the verge of taking over as coach at NAC Breda. However, a row with the Dutch Football Association (KNVB) about qualifications caused complications and the appointment fell through.