Dr Mike Roe, 33, of London, midwife Sara Nam, 30, of Carmarthen, west Wales, and transport expert David Heed, 26, of Wellingborough, Northamptonshire, who were all working for the charity Merlin, and Peter Colenso, 28, of Surrey, working for the US-based International Rescue Committee (IRC), were released at around 10.20pm British time, according to aid agencies.
Last night Mr Colenso's parents, Joan and Michael, said: "We are greatly relieved that Peter has been safely released. We have been enormously supported during this trying period by the IRC and the Foreign Office and others who have worked tirelessly to gain the release of the hostages.
"We are looking forward to speaking with Peter as soon as he can get to a telephone. We are very proud of him and the work he does." Former Eton pupil Mr Colenso, an Oxford University and London School of Economics graduate, was believed to be have been held on the Guinea side of the border, close to the town of Macenta, along with the other hostages.
Earlier, a three-man team of British diplomats led by Haydon Warren-Gash, the British ambassador in Abidjan, capital of the neighbouring Ivory Coast, had travelled to Liberia yesterday to assist the Liberian government in an effort to help free all six of the confirmed Western hostages.
The two other Western aid workers held by the gunmen were also freed. Alex Parisel, director of the Belgian branch of Doctors Without Borders, said two of his employees, together with the British workers, had all been freed.
"They have been well treated, they're safe and sound," he said last night.
Merlin said it had no official confirmation of the release of its workers but according to Mr Parisel, Merlin's officials said all of its employees had been accounted for.
Mr Parisel said the insurgents had made no demands to free their hostages, and appeared to want to release them as soon as they could.
"We have the feeling that our people were caught in the middle of a fight," he said.
The United Nations refugee agency, the UNHCR, said the gunmen could be detaining up to 100 people, around 50 international staff of non-governmental organisations and 50 of their dependants.
A UNHCR official in Ivory Coast named the abductors as the Joint Forces for Liberation of Liberia and said they were demanding fuel and other help in return for releasing the captives, who were seized in northwestern Liberia's Lofa county early on Wednesday,
The kidnapping was the latest in a series in West Africa that have also seen Britons seized in Sierra Leone and Nigeria.