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* FTSE 100 down 0.1%, FTSE 250 flat

Aug 4 (Reuters) - UK's FTSE 100 opened lower on Friday, with mining firms down and WPP falling after reducing its full-year outlook, and was on track to post its first weekly loss in four.

By 0713 GMT, the blue-chip FTSE 100 fell 0.1%, while the more domestically-focussed FTSE 250 midcap index was flat.

WPP slipped 5.7% after the world's biggest advertising group downgraded its full-year like-for-like growth forecast to 1.5%-3.0% from 3%-5%.

The broader media index was down 1.0%.

Industrial metal miners lost 0.7% as copper prices traded in a tight range.

The internationally-focussed FTSE 100 was headed for weekly losses as stocks were rattled this week from downbeat earnings and a surprise U.S. credit rating cut.

Capita slumped 8.3% after the outsourcing firm said it expected net exceptional costs of up to 20-25 million pounds ($31.8 million) related to the cyber incident in March but the group retained its full-year earnings outlook. (Reporting by Shashwat Chauhan in Bengaluru; Editing by Varun H K)