Shares in Paddy Power Betfair fell by almost 5% after the bookmaker said that chief executive Breon Corcoran was standing down.
The company also said that Mr Corcoran would be replaced by Worldpay's UK chief executive Peter Jackson.
Paddy Power said its half-year results, due on Tuesday, would show a 9% rise in revenues while underlying earnings are up 21%.
Underlying earnings for 2017 as a whole are expected to be £445m-£465m.
Paddy Power Betfair was the biggest faller on the FTSE 100, but the index itself managed to produce a small gain, jumping 0.3% to close at 7,531.94.
The index was boosted by mining companies, which saw their shares rise on the back of higher prices for metals. Shares in Glencore and Rio Tinto rose 2.6%, while BHP Billiton jumped by 2.3%.
In the FTSE 250, shares in QinetiQ rose 3% after Goldman Sachs raised its rating on the defence technology company to "neutral" from "sell".
On the currency markets, the pound fell 0.15% against the dollar to $1.3017, and fell 0.3% against the euro to 1.1045 euros.