However, Michael Orr, Molins' chairman, said the decision did not imply Mr Harrisson was responsible for the company's recent troubles. "We wouldn't seek to lay the blame for any of the problems we've experienced at his door," he said. "They are more a question of collective responsibility as well as external factors."
He said the resignation was down to "a difference in management style". Mr Harrisson will continue to be available to the company on a consultancy basis for the next three months.
Mr Harrisson, who was on a two-year contract earning pounds 160,000 a year, will receive a payoff worth about pounds 200,000. His share options are thought to be worthless.
Molins said Peter Grant, the finance director, will take on the chief executive's role "for the time being". However, he is also a candidate to fill the job permanently.
News of Mr Harrisson's departure prompted Molins shares to fall 12.5p to 270p, a five-year low. Brokers said the fall was prompted by fears that his departure was a sign that more bad news was on the way. But Mr Orr said the resignation had nothing to do with trading conditions.
The process of finding a replacement for Mr Harrisson has already begun, and a list of candidates has been drawn up. Analysts said a new face would reassure the City. "An outside appointment would be the best thing for Molins," one expert said. "But, more than anything, they need an appointment sooner rather than later."
- Peter Thal Larsen