Shares in Paddy Power Betfair fell by more than 5% after the bookmaker said that chief executive Breon Corcoran was standing down.
Mr Corcoran is being replaced by Worldpay chief executive Peter Jackson.
Paddy Power Betfair also said its half-year results, due on Tuesday, will report revenue growth of 9% and a 21% increase in underlying earnings.
It added that it expected underlying earnings for 2017 as a whole to be between £445m and £465m.
Paddy Power Betfair was the biggest faller on the FTSE 100 in early trade, but the index was up 18.82 points at 7,530.53.
The index was boosted by mining companies, which saw their shares rise on the back of higher prices for metals. Shares in Glencore, Anglo American and BHP Billiton were all up by about 1.6%.
In the FTSE 250, shares in QinetiQ rose nearly 6% after Goldman Sachs raised its rating on the defence technology company to “neutral” from “sell”.
On the currency markets, the pound was little changed against the dollar at $1.3041, and was also flat against the euro at 1.1075 euros.