The men faced a walk of five or six miles through rebel-held territory before reaching a checkpoint manned by Nigerian peace-keepers. Only then could their release and safety be guaranteed, said the source.
It was believed that some of the 20-odd hostages still held by the Armed Forces Revolutionary Council would remain in captivity after the release of the Britons, which was negotiated by a special British-United Nations mediation team.
Earlier the intermediary who brought back 19 hostages from the rebel stronghold in the Occra Hills on Sunday had returned "hopefully to get the rest", according to the Foreign Office. The three British officers, part of the United Nations observer mission in the West African country, were held with up to 24 other people. Among those released earlier were two British officers, and it became clear that the original group of those abducted had numbered nearly 50.
They were taken last Wednesday during an observer mission to the Occra Hills where they were due to oversee the handover to aid workers of several hundred child prisoners.
The hostages were held by a group from the Armed Forces Revolutionary Council (AFRC), which wants to know that its leader, Johnny Paul Koroma, is not a prisoner of the main rebel group, the Revolutionary United Front (RUF).
According to some reports, which were not confirmed by the Foreign Office spokesman in Freetown, the AFRC was sending four men to Monrovia, the capital of neighbouring Liberia, to see Mr Koroma. The hostage-takers were believed to be asking for guarantees of Mr Koroma's freedom and their own safety, perhaps in the form of a letter from the Sierra Leone President, Ahmed Tejan Kabbah. They want better recognition under a peace deal for Sierra Leone, which they feel favours the RUF.
The three British majors who were released last night were being held with one other United Nations military observer, believed to be from Kyrgyzstan. There are also up to 19 Nigerian peace-keepers with them and one civilian.
On Sunday, among the 19 people released, were Britons Lt-Col Ian Howard- Williams and Major Justin McEwan. They were taken with other released military to Marine House, the villa of the United States Marines, in Freetown.
Yesterday, Michael O'Flaherty, from the UN Human Rights Commission, said there was "no evidence of physical harm" having been done to the men.
"They have all been given access to medical assistance and trauma support," he said.