The warning was prompted by the cancellation of a lucrative agrochemicals contract by one of the firm's major customers, the US chemical giant DuPont.
David Wilbraham, Hickson's chief executive, said demand for agrochemicals had already been weakened by adverse conditions for farmers worldwide.
Sales in Hickson's organic chemicals division, for which the US group is the main customer, were worth pounds 29.5m in the first half of this year and pounds 84m for the same period in 1998. Yesterday's announcement fuelled speculation that Hickson would become a takeover target.
Hickson reported pre-tax profits of pounds 3.4m for the six months to 30 June, down from pounds 5.6m for the same period last year.