National Grid and WPP shed 5.08% and 3.46% respectively after ending dividend payments to shareholders.
Overall, the benchmark FTSE 100 share index was down 91.22 points, or 1.31%, at 6859.24.
Outside the 100-share index, price comparison websites Moneysupermarket and Zoopla fell 9.9% and 5.32% on fears of a competition probe.
Energy watchdog Ofgem says it is looking into "whether two or more companies providing a supporting service for the energy industry have breached competition law".
Low-cost airline Easyjet was among the top gainers, rising 0.63% on the strength of its latest passenger numbers.
The airline said it carried 7.2% more passengers in May than in the same month in 2014.
On the currency markets, the pound rose 0.13% against the dollar to $1.53600 and 0.22% against the euro to â‚¬1.36370.
(Close): London's leading shares closed lower on Thursday dragged down by companies going ex-dividend.