UPDATE 1-Nigerian militants say British hostage to be freed

* Militants say to free Briton after 9 months in captivity

* Last of more than 20 people seized in Sept

* Security forces avert attack on Shell pipeline

(Adds details, background)

By Randy Fabi

ABUJA, June 1 (Reuters) - Nigeria's main militant group said on Monday it would release a British oil worker held hostage in the Niger Delta for the past nine months.

"Today, June 1 is Matthew Maguire's birthday. He has spent close to nine months in captivity and we hope to release him today as his gift," the Movement for the Emancipation of the Niger Delta (MEND) said in an e-mailed statement.

Maguire has been held captive in the creeks of the Niger Delta, home to Africa's biggest oil and gas industry, since Sept. 9. MEND released another Briton seized with him, Robin Barry Hughes, in April because he was ill.

The militant group has hinted in the past it would release Maguire, but failed to follow through.

The two Britons were among more than 20 people taken when their oil supply vessel was hijacked.

Hundreds of foreigners have been taken in the delta since MEND launched a campaign of violence in early 2006 to push for what it considers to be a fairer share of the profits from crude oil extraction. Most are freed unharmed after a few weeks.

The military last month launched its most aggressive offensive against militants in the delta in years, bombarding militant camps around Warri in Delta state from the air and sea and sending three battalions of soldiers to hunt down rebels believed to have fled into surrounding communities.

In response, MEND has declared an "all-out war" against the military and bombed a Chevron (CVX.N) pipeline last week, forcing a loss in production of 100,000 barrels per day (bpd).

But there has been relatively little fighting between the two sides in the past week.

Colonel Rabe Abubakar said on Monday security forces averted an attempted militant attack on a pipeline connected to a Royal Dutch Shell (RDSa.L) platform in southern Nigeria's Rivers state.

"The troops who were deployed at the Shell platform were on their normal routine check of the area when the vandals were discovered," he said.

Attacks by MEND in the last three years have capped Nigeria's oil production at nearly 2 million bpd, significantly lower than its estimated 3 million bpd supply capacity. (For full Reuters Africa coverage and to have your say on the top issues, visit: africa.reuters.com/ ) (Editing by Tume Ahemba and Mark Trevelyan)