Britain's FTSE slumps to lowest closing level since Nov 2012

* FTSE 100 at lowest closing level since late 2012

* FTSE ends down 1.9 pct at 5,804.10 points

* Fears over global economy hit oil price

* But Randgold gains as gold price rises

By Sudip Kar-Gupta

LONDON, Jan 15 (Reuters) - Britain's top share index fell to its lowest closing level in more than three years on Friday, hit by losses in commodity-related stocks as BHP Billiton (NYSE: BBL - news) suffered a major writedown and oil fell below a key level.

The blue-chip FTSE 100 equity index ended down 1.9 percent at 5,804.10 points - its lowest closing level since late 2012.

The FTSE is some 20 percent below a record high of 7,122.74 points reached last April (LSE: 0N69.L - news) and 7 percent down since the start of 2016.

Concerns over China, the world's second-biggest economy and biggest consumer of metals, have hit oil and metals prices.

Oil fell below the $30 level on Friday, knocking down shares in BP and Royal Dutch Shell (Xetra: R6C1.DE - news) , while copper prices also slid to their lowest level since May 2009.

Mining stocks suffered a further blow after BHP Billiton said it would write down the value of its U.S (Other OTC: UBGXF - news) . shale assets by $7.2 billion due to the bleak outlook for oil prices, increasing concerns it will be forced to cut its dividend for the first time in over 25 years.

BHP Billiton fell 6.4 percent, while Anglo American (LSE: AAL.L - news) slumped 11.5 percent. Glencore (Xetra: A1JAGV - news) also fell 6.5 percent.

"This is a commodity-driven fall on the markets. Oil has gone below $30, and that's added to fears. It (Other OTC: ITGL - news) 's very difficult to call the bottom of this market," said Thames Capital Markets' trader Daniel Woodward.

Price comparison site Moneysupermarket, which is in the FTSE 250 mid-cap index, plunged 10.7 percent after reporting a bigger-than-expected fourth-quarter contraction in its insurance unit.

However, gold miner Randgold rose 3.7 percent.

Randgold was the best-performing stock on the FTSE 100 as it benefited from a rise in the price of gold itself, which typically gains in value when there are fears about the global economy, due to gold's status as a safe-haven asset. (Additional reporting by Kit Rees; Editing by Hugh Lawson)