With West Brom on the verge of relegation from the Premier League, Johnstone is likely to leave the Baggies in the summer for another top-flight team.
And he is on the shortlist of several clubs in the Premier League who are keen to poach him from the Hawthorns in the next transfer window.
Tottenham have been keeping tabs on him and it has been claimed that Manchester United manager Ole Gunnar Solskjaer wants him to replace David de Gea as the club’s number 2 next season.
But West Ham have emerged as the favourites to sign the goalkeeper during the summer transfer window.
West Ham manager David Moyes worked with him at Manchester United, where Johnstone spent nine years and is prepared to offer him regular football.
With Lukasz Fabianski’s contract expiring at the end of next season, the Hammers are looking at the West Brom goalkeeper as the Pole’s long term replacement.
A return to Manchester United could be an emotional one for Johnstone but he is being looked at as Dean Henderson’s deputy.
Tottenham are also believed to be keen on him as the number 2 to Hugo Lloris at the moment.