A Briton kidnapped in Nigeria last month has been killed with three others released after negotiations, Britain's Foreign Office said on Monday.

The four Britons were taken hostage three weeks ago and although the British High Commission, with help from the Nigerian authorities secured the release of three of them, Ian Squire was killed, the Foreign Office (FCO) said in a statement.

It said it was unable to give further details because of an ongoing investigation by the Nigerian authorities

"We are supporting the families of four British people who were abducted on October 13 in Nigeria, one of whom was tragically killed," the statement said.

"This has clearly been a traumatic time for all concerned, and our staff will continue to do all we can to support the families."

A statement from the families of Squire and the other hostages, Alanna Carson, David Donovan and Shirley Donovan, said the last three weeks had been traumatic for the relatives and friends of those involved.

"We are delighted and relieved that Alanna, David and Shirley have returned home safely. Our thoughts are now with the family and friends of Ian as we come to terms with his sad death," their statement said.

Kidnapping for ransom is rife in southern Nigeria, where high-profile individuals and their families are a frequent target for criminal gangs.

Victims are usually released after a few days once payment is made.

Militants suspected

Nigerian police believed militants who have attacked oil and gas pipelines in the oil-rich Niger delta region were behind the kidnapping.

They said on October 19 that four people had been arrested in connection with the abduction.

Nigerian intelligence sources said the Britons were thought to have been taken to militant camps in the creeks and swamps of the delta.

The four had been providing "free medical care and religious activities" in the Burutu area of Delta state, said Chief Theo Fakama, from the local Enukorowa community.

Fakama said locals were "saddened" by the kidnapping as the group had "brought succor to residents of the community for the past three years".

Delta state police spokesman Andrew Aniamaka said on October 18 that the Britons had been providing humanitarian services.

"But unfortunately, they didn't let the authorities know of their presence in the area all this while," he said.

"There is a militant group that has been operating in the area and we believe they are the ones behind the abduction."

Last month, an Italian priest based in Nigeria for the last three years was kidnapped by armed gunmen near Benin City, the capital of Edo state. He was released within days.