It is believed that the Royals would like to sign Johnson initially on loan with a view to a permanent deal, but Birmingham are said to be holding out for a full transfer, and a £250,000 fee.
Reading had hoped to have the defender in time for tomorrow's game against Ipswich, unfortunately that will now not happen.
Stoke, Derby, and Walsall are also alleged to have shown an interest in the player Birmingham fans nickname 'Magic'.
The 30-year-old Jamaican international is said to be keen on a move away from St Andrews as his first team chances are severely limited. Indeed, the veteran made just six appearances last season.
Steve Bruce has laid his faith heavily in the sublime Upson-Cunningham partnership in recent months. As Reading fans will testify, there is no shame in being kept out of the first team by Matthew Upson.
Johnson, a left-footed defender who plays predominantly at centre-back, has spent the last eight years at St Andrews. The Nottingham born player was originally brought to Birmingham by Barry Fry, who paid Notts County £225,000 to seal the deal.
The 30-year-old has made 256 appearances during his Birmingham City tenure, scoring 13 goals. He was awarded with the club's Player of the Year award in 1998-99.