By Sara Sjolin, MarketWatch

LONDON (MarketWatch) -- Shares of Intertek Group PLC slumped on Friday after cautious guidance from the product-testing company, leading the U.K.'s FTSE 100 index lower for a second straight day.

The benchmark index fell 0.1% to 6,834.45, trimming its weekly gain to 0.3%.

Intertek slid 5.5% after the firm said market conditions remain variable "with good growth in our key product-related business lines, but weak demand in certain other markets."

Mining firms also weighed in London as metals prices were mixed. Shares of Rio Tinto PLC (RIO) lost 1.8%, Anglo American PLC fell 1.6% and BHP Billiton PLC (BHP) gave up 1%.

More broadly, developments in Ukraine also kept investors on edge on Friday. Bloomberg News reported that Ukrainian troops have moved to force separatists from their eastern rebel bases near the towns of Slovyansk and Kramatorsk, citing acting President Oleksandr Turchynov. Meanwhile, U.S. and U.K. diplomats vowed to impose further industrywide sanctions on Russia if the country undermines Ukraine's presidential election later this month.

On a more upbeat note in London, U.K. supermarket chains capped losses for the FTSE 100. Shares of Wm. Morrison Supermarkets PLC gained 4.6%, J Sainsbury PLC climbed 2.6% and Tesco PLC added 2%.

More must-reads from MarketWatch:

Virtual Kim Jong Un rides a unicorn into battle against U.S. troops

Too-big-to-fail battle between Larry Summers, Nassim Taleb

The 10 companies with the best corporate cultures

Subscribe to WSJ: http://online.wsj.com?mod=djnwires