Portsmouth executive chairman Peter Storrie has confirmed that the club have received a bid for Johnson.
He told Sky Sports News: “Glen has had a fantastic season for us and I think he is the best right-back in the country at the moment.
“I’ve know Glen a long while, from my West Ham days. When I re-signed him (at Portsmouth) we talked things over.
“He said, ‘If one of the big four come in with Champions League football, would you stand in my way?’
“I told him I couldn’t do that. Providing the deal is right for Portsmouth and Glen then we will look to try and do something in the right way.
“It is very hard to try and hold people back when they have a chance to play in the Champions League. I accept that.”