The blue-chip FTSE 100 and the domestically focussed mid-cap index slipped 0.2% each by 0706 GMT, tracking weakness in their European and Asian peers.
Banks slipped 0.7% while consumer staples such as Diageo and Unilever fell 0.8% and 0.5%, respectively, and weighed most on the benchmark index.
Capping losses on the commodity-heavy index, oil majors BP and Shell climbed 0.7% and 1%, respectively, as crude prices rose on supply concerns. [MET/L]
Worries of a global economic slowdown lingered ahead of an expected 75-basis-point interest rate hike by the U.S. Fed around 1800 GMT, weighing on commodities and the broader market.
Financial markets expect the Bank of England to follow suit on Thursday, pricing in a 73.4% chance of a 75-basis-point increase in rates to 2.5%, and a 26.6% chance of a smaller 50-basis-point increase. [IRPR]
The sterling briefly dipped to its lowest against the U.S. dollar since 1985 after data showed Britain's budget deficit was bigger than expected in August.
Shares of BAE Systems gained 3.7%, tracking upbeat defence peers in Europe, after Russian President Vladimir Putin ordered Moscow's first military mobilisation since World War Two.
(Reporting by Bansari Mayur Kamdar in Bengaluru; Editing by Sherry Jacob-Phillips)