The UK energy index dropped 1.2 percent as the oil price fell for a fourth consecutive session after Iran insisted it would not freeze crude output, returning investor attention to a global glut. Shares in BP (>> BP plc) and Royal Dutch Shell were down more than 1 percent.

The mining index also came under pressure, edging down 0.3 percent, after copper slipped towards a three-month low as expectations that the U.S. Federal Reserve will raise interest rates in June reinforced worries about weak demand growth in top consumer China. BHP Billiton (>> BHP Billiton plc) fell 0.8 percent.

"The two sectors could face further selling pressure in the near term as sentiment towards commodities prices remains fragile," Securequity senior trader, Jawaid Afsar, said.

"There are numerous headwinds such as a stronger dollar and lingering concerns about the pace of economic growth in China, Europe and the United States," he said.

The blue-chip FTSE 100 index <.FTSE> closed 0.3 percent lower at 6,136.43 points. Losses were mitigated by a 4.3-percent gain in Royal Mail (>> Royal Mail PLC) shares after upgrades from brokers Cantor Fitzgerald and RBC Capital Markets.

"We expect parcels in the UK and Europe to drive the top-line and restructuring and productivity gains to support the bottom-line," Cantor Fitzgerald analysts said in a note.

"(Royal Mail) faces potentially tough wage and pension negotiations this year but the risks are probably overstated," they said.

Among mid-caps, sports retailer Sports Direct (>> Sports Direct International Plc) dropped 4 percent after Goldman Sachs downgraded the stock to "neutral" from "buy".

"The UK high street is changing - we're seeing quite an uptick in online shopping," Charles Hanover Investments advisory investment manager, Jonathan Roy, said.

"The retail space that (Sports Direct is) occupying is becoming a bit of a burden and that's a trend that we expect to see follow through for the rest of 2016 and beyond."

(Additional reporting by Atul Prakash; Editing by Louise Ireland and Hugh Lawson)

By Kit Rees