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* FTSE 100 down 0.6 pct
* FTSE 250 down 0.7 pct
* Miners, exporter shares drag on main index
* Metro Bank hits life-low after Q1 results
May 2 (Reuters) - A fall in blue-chip copper miners and a firmer sterling drove the FTSE 100 lower on Thursday, while mid-cap Metro Bank tanked after an accounting error caused quarterly profit to halve.
The FTSE 100 was down 0.5 percent and the FTSE 250 was 0.7 percent lower by 0730 GMT.
Sterling gained after investors took comments made by Prime Minister Theresa May as an indication of progress in Brexit talks between the government and the main opposition Labour Party.
This led to shares of dollar earners such as British American Tobacco and Diageo shedding more than 1 percent.
Copper prices hit a more than two-month low, causing an index of miners to fall for the seventh consecutive session and leading heavyweights Rio Tinto, Glencore and BHP more than 2.2 percent lower.
But medical products maker Smith & Nephew outperformed the main bourse with a 2.9 percent gain, as it forecast annual revenue growth at the top half of its prior guidance range.
On the mid-cap index, Metro Bank slumped 16 percent to an all-time low after saying the fallout from an accounting error it disclosed in January hit quarterly profit, capital buffers and major business customer deposits. (Reporting by Shashwat Awasthi in Bengaluru Editing by Gareth Jones)