LONDON (Bloomberg) ? Benfield Group Plc, a UK reinsurance broker, said as many as 20 workers in one unit have resigned to join rival Aon Corp., cutting this year?s profit by ?10 million ($18.55 million).

The resignations mean the company ?is no longer confident? of an earlier revenue forecast for the last quarter of 2006, said Benfield, which is domiciled in Bermuda but has its main headquarters in London.

Twenty employees among 110 working on the Facultative Solutions team have left, including team head Elliot Richardson, spokesman David Bogg said by telephone yesterday. Benfield?s Bermuda office offers facultative solutions, among other services.

?We have started a process to take appropriate legal steps,? said Bogg. ?We have been advised that people have left, which gives rise to a claim against them,? he said.

Chicago-based Aon, the world?s second-biggest insurance broker, is hiring workers away from one of the more-profitable segments of Benfield. The unit, which specialises in reinsuring one-time risks and represents less than six percent of total revenue, accounts for a greater share of profit, Benfield said. The company?s first-half profit rose 43 percent to ?64 million, it said in September.

Shares of Benfield fell eight percent, the biggest one-day decline in more than a year, to 347 pence at the close in London, giving the company a market value of about ?811 million.

?The market?s reaction is greater than the financial impact the departures will have,? said Nick Johnson, a London-based analyst at Numis Securities Ltd. He rates the stock ?hold? and will change that to ?add? if the price falls below 345 pence. ?It is perhaps fearful that this is an indication of previously unapparent instability of the staff there,? he said.

Richardson is leaving Benfield to be Aon?s London-based global head of facultative reinsurance, Aon said on September 25. The company has been recruiting in the ?facultative reinsurance area for some time?, it added at the time.

Aon has no further comment, said London-based spokesman James White by telephone today.

Aon?s second-quarter profit rose 1 percent to $193 million, it said in August. Profit was ?driven by reinsurance growth, which has been negative in the past,? it said.

Benfield believes most of those resigning at the specialist unit have joined Aon, Bogg said. Richardson ?has left to pursue his career with a competitor,? Bogg said. ?We are reviewing our legal position with regard to Aon,? he said.

Benfield has ?already put in place measures to strengthen leadership? of the unit, it said. The company?s main property and casualty reinsurance broking operation ?continues to benefit from current market conditions,? it said.

Benfield is boosting US revenue after gaining clients that switched brokers last year following a regulatory investigation of industry business practices. Demand for catastrophe reinsurance has increased and rates have risen after record storms last year.