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* FTSE 100 down 0.9 pct
* Mid and small caps outperform
* Anglo American, Glencore, Rio among top fallers
* Iron ore slump weighs on Ferrexpo
* BP falls on lower crude prices
By Helen Reid
LONDON, April 18 (Reuters) - British shares followed European markets down on Tuesday, set for their worst day's fall in three months as lower metals and crude prices weighed on the commodities-heavy index.
The FTSE 100 hit a seven-week low, down 0.9 percent, though it pared some losses after sterling fell to a day's low as investors awaited a statement by Prime Minister Theresa May.
Sterling's weakness combined with stronger than expected fundamental economic data had supported the blue-chip index to record highs in the past months.
"We have a bit of reluctance to chase things much further, until you get another round of evidence," said Ian Williams, economics and strategy analyst at Peel Hunt, adding that strong results in the U.S. first-quarter earnings season would be required to push markets higher.
Mining companies Anglo American, Glencore, Antofagasta, Rio Tinto and BHP Billiton were top fallers, down as Chinese iron ore futures fell to their lowest level in three months, with oversupply worries weighing on steel prices.
Oil major BP was also a top faller, down 2.8 percent as the price of crude fell. Brent crude reached an eleven-day low after a U.S. government report indicated rising production.
The mid caps and small caps outperformed the blue chips, down 0.6 and 0.2 percent respectively, and holding near all-time highs hit in the last trading session.
"I think people have been pleasantly surprised by how sanguine companies' management teams have been," said Williams. "In the last six weeks the mood of companies presenting has been much more optimistic, and people are asking whether some of the risk has been priced in already."
The top fallers on the mid-cap index were also all commodity-linked stocks, led by iron ore pellet producer Ferrexpo, down 5.2 percent, and miner Evraz down 6.3 percent.
Among the few risers in mid and small-cap stocks were shipping services providers James Fisher and Braemar Shipping Services, up 2.4 and 3.3 percent.
Emerging markets-focused fund manager Ashmore was down 2 percent, despite earlier posting net inflows for the first time in nearly three years, as investors turned back into developing regions.
Reporting by Helen Reid; Editing by Angus MacSwan