Having established himself in both the Portsmouth and England set-ups, Johnson signed a new four-year deal with Pompey earlier this year but club officials accepted they could not stand in his way if he was offered the opportunity to move to a club playing in the Champions League.
Portsmouth chief executive Peter Storrie said last week: "Glen has obviously had a fantastic season season for us and has established himself as one of the best right-backs in the league if not the country at the moment.
"I've known Glen for a long time since his West Ham days so when I re-signed him we talked it over and he asked if one of the big four clubs, who were in the Champions League, came in for him would we stand in his way of a move.
"I said that I couldn't do that provided the deal is good for Portsmouth and for Glen."
