* UK faces unusually close national election in May

* Energy prices a politically sensitive issue

* Labour wants to show voters it is on consumers' side

By Andrew Osborn

LONDON, Jan 11 (Reuters) - Britain's opposition Labour party said it would try to introduce a law to compel the country's energy regulator to force firms to cut prices in response to any fall in wholesale costs, a move it hopes will boost it four months before a national election.

The plan, announced on Sunday by Ed Miliband, the party's leader, will see Labour bring forward a vote on a motion to fast-track such legislation before parliament on Wednesday.

Miliband called on the governing Conservatives and Liberal Democrats to support the new law. Prime Minister David Cameron has balked at similar ideas before, portraying them as meddling in the free market.

"We've seen wholesale costs go down 20 percent in gas prices over the last year and no reduction in bills," Miliband said.

"We'll give the regulator the power to cut prices to bring immediate relief," Miliband told BBC TV's Andrew Marr Show, referring to Ofgem.

Labour's move is meant to appeal to voters, many of whom have felt their living standards squeezed by inflation outstripping wages until recently, before what is shaping up to be one of the closest elections in decades.

A sharp drop in oil prices has led to a fall in petrol prices in Britain and politicians are seeking to court voters by competing to sound tougher on firms perceived to be not cutting their prices fast enough.

A 2013 proposal by Miliband to freeze gas and electricity prices for 20 months gave his party a temporary lift in opinion polls. It was criticised by the country's big six energy suppliers and business groups.

Conservative finance minister George Osborne last week said it was vital that the lower oil price be passed onto families "at petrol pumps, through utility bills and air fares". Danny Alexander, the Liberal Democrats' most senior figure in the same ministry, has made similar comments.

While Cameron's Conservatives want to fight the election on their record of presiding over an economic recovery, Labour want to use the energy question, among others, to highlight what they say has been a cost of living crisis, a charge they hope will blunt the Conservatives' economic boasts.

Miliband on Sunday called for the introduction of a new benchmark, an "independent Living Standards Index", to measure people's well being.

An opinion poll released by YouGov on Sunday had Labour and the Conservatives tied on 32 percent, the Liberal Democrats on 7 percent, the United Kingdom Independence Party on 18 percent, and the Greens on 6 percent.

(Editing by Stephen Powell)