Blue chip shares fell in afternoon trading as a number of heavyweight stocks went ex-dividend, making them less appealing to investors.
The FTSE 100 index fell 25.7 points to 7,276.7, as such stocks as BP and AstraZeneca went ex-dividend. This means traders who buy now will not be eligible for the firm's next dividend, which pushes these stocks lower. The FTSE 250 was 91.7 points lower at 18,735.5.
IG chief market analyst Chris Beauchamp said: "BP, Shell, AstraZeneca and Imperial Brands are all trading without the benefit of the dividend, and with these heavyweights in the red it is going to be tough for the market to push higher."
He added: "Given the steady rally we have seen over the past week or more, however, some weakness is to be welcomed, taking a little heat out of the market."
The move came as the MSCI World Index, which tracks shares in 46 markets, climbed to an all-time high of 444.29.
In London's second tier, Cobham plunged after the UK aerospace group warned on profits for the fifth time in less than two years.
Shares dropped as much as 21% in early trading after the firm said annual underlying profits would be £20m lower than forecast, while a number of "exceptional" charges will drag its reported figures even lower. The stock recovered slightly in later dealing.
Cobham was 19.6p down at 115.8p.
The biggest afternoon risers in the FTSE 100 Index were Coca-Cola HBC (+60p to 1,883p), Shire (+131p to 4,720p), Capita (+11.5p to 519p), Smurfit Kappa (+42p to 2,201p) and International Airlines Group (+6.5p to 510p).
The biggest afternoon fallers in the FTSE 100 Index were AstraZeneca (-168.5p to 4,515.5p), Anglo American (-48.5p to 1,346.5p), Antofagasta (-21p to 845p), Imperial Brands (-73.5p to 3,626.5p) and BP (-7.5p to 450.1p).
The biggest afternoon risers in the FTSE 250 Index were Lancashire Holdings (+59p to 739.5p), OneSavings Bank (+10.1p to 377.4p), Shawbrook Group (+6.1p to 266.1p), Vectura Group (+3p to 144.6p) and Daejan Holdings (+135p to 6,600p).
The biggest afternoon fallers in the FTSE 250 Index were Cobham (-19.6p to 115.8p), Drax Group (-27p to 352p), Brewin Dolphin (-10p to 306.5p), Meggitt (-12.5p to 422.5p) and SIG (-3p to 109.8p).