Mr Johnson is thought to have lost out in a struggle with Charles Brady, the American who took over as chief executive from Lord Stevens in July. His exit will bring down the curtain on a disastrous year for Invesco MIM.
Mr Johnson has spent virtually his entire career with the group, which he joined in 1976. He was promoted to the board in 1987 and, until a reshuffle in July, was a joint deputy chairman with Mr Brady.
It is thought that Mr Johnson, who holds 1.5 million share options and earns around pounds 200,000 a year, will be entitled to a substantial pay-off.
Mr Brady divides his time between London and Atlanta, where he runs Invesco's highly profitable US fund- management operation, which specialises in investing pension scheme money. This business has been untouched by the catalogue of problems besetting the group in London.
The American executive was promoted over Mr Johnson's head in July when Lord Stevens, who plans to retire completely from Invesco MIM next June, gave up day-to-day control while remaining as non-executive chairman.
The July reshuffle followed months of damage for Invesco MIM. The group was badly hit by the dismal showing of Drayton Consolidated, the investment trust then managed and partly owned by Invesco MIM and formerly chaired by Lord Stevens. The debacle led to Invesco MIM making an unprecedented pounds 9.5m cash payout to compensate investors in Drayton. Mr Johnson is thought to have had no involvement with the Drayton disaster.
The group has also suffered adverse publicity over its role as one of the managers of the pension funds looted by the late Robert Maxwell, a long- time business associate of Lord Stevens. Trustees of the Mirror Group pension scheme sued Invesco MIM for the return of more than pounds 11m - a claim it is resisting.
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