Fifteen oil and gas workers, including seven Britons, have been freed after being taken captive on a survey vessel off the west coast of Africa.

The Foreign Office said the crew were held by authorities in Equatorial Guinea for about three weeks after their ship allegedly strayed into the country's waters illegally.

The team, which also included a Belgian, a South African, three Nigerians and three Ukranians, had been working aboard the survey vessel MV Askelad for the engineering firm Gems UK.

The vessel is owned by the UK-based Thales Group, which is a major supplier of electronic equipment to the Ministry of Defence.

A Foreign Office spokeswoman could give no details last night of the Britons involved. She said British consular officials in Lagos, Nigeria were involved in negotiating the group's release.

Father-of-two Chris Wood, 44, a survey engineer from Great Yarmouth, Norfolk, was one of the Britons taken captive at gunpoint.

He and other members of the group were taken to the Tropicana Hotel in Malabo, the island's capital, where they were detained.

On hearing the news that he had been freed, Mr Wood's wife, Belinda, 41, said: "I got an e-mail from my husband just before 10am today to say they'd got back on board their vessel.

"They're going back into Nigerian waters. I can't stop smiling, it's such a relief."

Mrs Wood added: "They were only supposed to be held for a few days so it was a bit worrying. Chris was starting to get low. We had no idea how long it would take."

Mrs Wood said her husband, who works as a freelance survey engineer, was being employed by a firm based in Wiltshire.

She added: "They've still got the work to do that they went out there for and they're not due back until the end of March."

The incident is the latest in a series of security scares involving oil workers in the Niger Delta area where militants have launched a wave of kidnappings and attacks on oil facilities.

In June 2006, six Britons were taken from an oil rig off Nigeria's south coast and released two days later.

The following October four Scots were among seven men seized from the Exxon Mobil compound in Nigeria. They were later freed. Hostages in the area are rarely armed and are usually released after money changes hands.

No-one from Gems UK of Thales could be reached for comment last night.