The blue-chip index, which has greater exposure to international markets, was down 0.7% - its biggest one-day drop in six weeks, with all except three of its 100 components in negative territory by 0705 GMT.
However, the index was supported by Flutter Entertainment , formerly known as Paddy Power, which jumped 13% to top of the FTSE 100 after agreeing to an all-share deal with Poker Stars to create one of the world's biggest online betting and gambling companies.
The more-domestically focussed FTSE 250 fell 0.4%, but gambling firms William Hill and GVC rose 3% and 5%, respectively, limiting overall losses.