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At 09:18 BST the index was down 0.2 per cent to 4,179.80 points, Reuters reports.
Oil companies led the slide, with Tullow Oil falling 4.1 per cent following the announcement that it expects half-year revenue to decrease.
BP, Royal Dutch Shell, BG Group and Cairn Energy also declined and market analyst Nick Serff explained that firms operating in this sector "are in the spotlight" following falls in the price of crude oil.
"Second line oil shares came under the most pressure with investor focus switching away from the larger cap stocks," he remarked.
Mining firms helped stabilise the market, with Rio Tinto, Antofagasta and BHP Billiton among the stocks that rose in value.
Mr Serff noted that investors will be paying close attention to the FTSE 100 ahead of the start of the second-quarter earning season, which begins later today.
