The beleaguered consulting group also issued a profit warning after a thorough review of contract risks and project claims.
According to new finance director Rod Harris, who joined in June from Carillion Business Services, the net impact could be around £8m.
Richard Cuthbert led Mouchel’s fight against take-over bids from both Costain and Interserve last year.
He argued that the consultant was in good enough shape to ride out its financial problems and provide long term shareholder value.
In a short statement this morning Mouchel said: “Richard Cuthbert has tendered his resignation with immediate effect, and he will work with the group for a short period to ensure an orderly handover.
“Bo Lerenius will become executive chairman until a new chief Executive has been appointed.”
In June, Mouchel reported that it expected a significant one-off profit on one of its long-term contracts to offset lower profitability than originally expected following commercial conclusions on a number of contracts.
Today it said: “The group was very recently informed that, as a result of an actuarial error, the one-off gain will be £4.3m lower than previously expected, which has an immediate and corresponding impact on the profits the group will report for the year to 31 July 2011.”
It also warned that the new finance director had reviewed contract risks and project claims, taking into account the continuing challenging business environment.
“As a result, the group has decided to increase provisions in respect of these contract risks and project claims by a similar amount to the reduction in the one-off gain.”
Bo Lerenius said: “I would like to express the board’s gratitude to Rich for his many years of service and his continuing belief in and commitment to the business.
“The board will focus on maintaining Mouchel’s position in its core markets in the interests of all its stakeholders.”