Four British missionaries kidnapped in Nigeria include Christian couple David and Shirley Donovan

The four British missionaries kidnapped in southern Nigeria are thought to include a couple from Cambridge who have worked in the country for 14 years.

Former GP Dr David Donovan and his wife Shirley were taken from where they were staying in rural Delta State around 2am last Friday, according to the Times. They run a medical charity called New Foundations providing aid to remote villages in the Nigerian delta.

Two other victims, known only as 'Alana' and 'Tyan' were also kidnapped by the gang, which is thought to be well known for making ransom demands.

Open DoorsNigeria is ranked 12th on a new list of countries where Christians face the most persecution.

Since the kidnapping the New Foundations website and a number of links to Donovan have been taken down and Michael Coleman, the UK director, declined to comment other than telling the Times he was working with the Foreign Office.

The charity began when the couple launched a boat clinic, serving remote villages along the creeks of the delta. Over a two-year period leading up to 2008 they carried out 500 cataract operations, treated more than 16,000 patients,  vaccinated more than 4,000 children and installed rain harvesting systems in eight villages.

Donovan is still linked to the Riverside GP Practice in Cambridge but is thought to have worked mainly in the Delta region of Nigeria for more than a decade.

The area has a history of separatist movements and has suffered a spate of kidnappings recently linked to the central government's crackdown on Islamist militants.

Zanna Ibrahim, the police commissioner for the state, said: 'The kidnap may not be unconnected with the present onslaught on militant activities embarked upon by the military, codenamed Operation Crocodile Smile.'

Andrew Aniamaka, from Delta State police, said: 'The abductors have not made any contact but we are doing our investigations to know the motive and have them rescued without jeopardising their lives.'

WikipediaDelta State is in the south of Nigeria and is known as 'the big heart of the nation'.

The UK Foreign Office has not commented but had advised against all but essential travel in parts of southern Nigeria, including the region from where the Donovans are thought to have been taken.

Kidnapping for ransom is a common problem in parts of Nigeria and high-profile individuals and Westerners are often the target. A number of foreigners have, in the last few years, been kidnapped in the Niger Delta region, which holds most of the country's crude oil – the country's economic mainstay.

Chief Theo Fakama, from the local Enukorowa community, told the Agence France-Presse news agency that villagers were saddened by the kidnapping because the victims had 'brought succour to residents of the community for the past three years'.