West, who told PRWeek he had resigned from the company, remains a shareholder.
Ridsdale and Simson have been appointed as joint chief executives and both will continue with their client-handling roles – Simon in the natural resources sector and Ridsdale in corporate, high-net-worth individuals and special situations.
Ridsdale joined Tavistock in 2005 and Simson in 2010, following its acquisition of Conduit PR which specialised in comms for natural resources companies.
Jeremy Carey, Tavistock’s chairman, said: "Since we founded the business more than 20 years ago we have maintained a record of developing talent and promoting from within and these appointments reflect that ethos."
One industry source, speaking anonymously, claimed that Tavistock's switch in focus towards larger projects was responsible for West's depature.
The source said: "It looks like the chief executive was pushed rather than walked and that reflects the changing nature of the business in respect of going for big-ticket stuff. Matt and Jos are the obvious choice for stepping up and leading that change."
However, West denied he had been asked to leave, saying: "It’s not true. It’s just people stirring things up."
Tavistock said West had resigned and had left "with its best wishes".