After the England goalkeeper allows his West Brom contract to run its course, Manchester United and Spurs will battle it out for his signature in the summer.
The player who has won three senior caps for England refused the offer of a new deal at the Hawthorns to return to the top flight.
A number of other clubs are thought to be interested.
Steve Bruce said at the weekend that he expects Johnstone to leave at the end of the season, rather than stay and fight for promotion.
Four years ago, officials at Manchester United allowed him to leave the club, but have kept a close eye on his progress.
Dean Henderson is considering his options as he seeks more regular football, and there could be a significant shake-up in the goalkeeping department this summer.
Last season, Henderson was vying with David de Gea for the United No 1 jersey, but this season he hasn't played in a match.
Henderson has three years left on his current deal, but there are question marks about which clubs would be willing to pay his full wages.
It is thought that United would like to sign him, and then consider Henderson's future.
Henderson and Johnstone are both desperate for regular top-flight football in order to reignite the interest of England's manager.
Spurs were close to a deal for Johnstone in January, but decided not to pursue their interest as they prioritised other positions in the squad.
Hugo Lloris signed an improved contract at the Spurs in January, and Antonio Conte wants to have more competition for him.
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