* FTSE 100 down 0.5 pct
* FTSE 250 down 0.6 pct
* Miners, exporter shares drag on main index
* Shell, Smith & Nephew outshine
* Metro Bank hits life-low after Q1 results (Adds news items, analyst comment, updates to closing prices)
By Shashwat Awasthi and Yadarisa Shabong
May 2 (Reuters) - The FTSE 100 ended lower for the third straight session as copper miners fell and exporter stocks struggled despite weakness in sterling, while Metro Bank tanked after an accounting error caused quarterly profit to halve.
Strong results from Shell and Smith & Nephew capped losses, but the FTSE 100 was still held to a one-month low, falling 0.5 percent. The FTSE 250 was 0.6 percent lower.
Sterling had initially gained on upbeat Brexit sentiment, but its rise was halted after the Bank of England voted unanimously to keep interest rates steady and warned Brexit continued to cloud the outlook for monetary policy.
"Another sobering Bank of England statement also added some gloom, despite policymakers' attempt to portray Britain's economy as straining at the leash of inflation," Cityindex analyst Ken Odeluga said.
Dollar earners such as British American Tobacco and Diageo, which had shed more than 1 percent earlier, were unable to overcome the deficit and closed lower.
Paddy Power Betfair was the worst blue-chip performer as it skidded 5.4 percent on unfavorable sports results in Britain and Ireland and pointed to a 43 percent drop in FOBT revenues.
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 about 2 percent lower.
But Shell, the biggest FTSE 100 company by market value, rose nearly 2 percent after beating quarterly profit expectations.
Medical products maker Smith & Nephew also outperformed the main bourse with a 3 percent gain, as it forecast annual revenue growth at the top half of its prior guidance range.
Retailers dived in seemingly no-news moves. M&S, Sainsbury's and Burberry all tumbled more than 2 percent.
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.
Among smaller stocks, cleaning products maker McBride sank 10.8 percent after its second profit warning this year.
Thomas Cook slumped 15 percent. A trader cast doubt on the tour operator's plans to sell its airline business. (Reporting by Shashwat Awasthi and Muvija M in Bengaluru; Editing by Angus MacSwan, William Maclean)