LONDON, Dec 21 (Reuters) - Royal Dutch Shell is shutting down its huge 200,000 barrels per day (bpd) Bonga oilfield off the Nigerian coast after a leak occurred while loading a tanker on Tuesday, the firm said in a statement.

The Anglo-Dutch oil major said "less than 40,000 barrels of oil" had leaked into the ocean. The flow of oil had now halted, a spokesman said.

The leak occurred while a tanker was loading oil from Shell's Bonga facility, about 120 kilometres off the coast of the West African nation, according to the statement.

Shell's pipelines in Nigeria's onshore Niger delta have spilled several times, which the company blames on sabotage attacks and oil theft.

Bonga accounts for around 10 percent of monthly oil flows from OPEC member Nigeria, the continent's largest exporter of crude oil, according to Reuters data.

"We are sorry this leak has happened. As soon as we became aware of it, we stopped the flow of oil and mobilised our own resources, as well as industry expertise, to ensure its effects are minimised," said Shell Nigeria Country Chair Mutiu Sunmonu.

"It is important to stress that this was not a well control incident of any sort, and to make clear that no one has been injured. Our focus now is on a speedy and effective clean-up," he added.

BP's Macondo well ruptured in April last year, causing nearly 5 million barrels of oil to spew into the sea in what was the worst U.S. marine oil spill.

Shell Nigeria Exploration and Production Company is 100 percent owned by the Anglo-Dutch oil major.

Flow halted

A Shell spokesman said the flow of oil had been halted on all three of the platform's export lines where it is believed that the leak occurred.

An investigation will be launched into the reasons for the leak, he said, without giving a timeframe for the restart of production.

The company had not declared force majeure, a legal clause allowing a company to miss deliveries due to circumstances outside its control, he added.

Shell's share price fell during the day by more than 1 percent on Wednesday to 2,274 pence by 1405 GMT. Brent oil prices were up 63 cents at $107.36 a barrel by the same time.

Oil traders and analysts said the Bonga closure could be supportive both for Brent oil futures and other Nigerian crude streams, where demand is expected to increase.

"Given the current sensitivity to potential supply disruptions and rising geopolitical tensions it just adds to the uncertainty in 2012... It will limit the downside on Brent should broader market sentiment turn," said Andrey Kryuchenkov of VTB Capital.

Bonga was due to load around 161,000 bpd on five tankers in January, according to oil loading programs.