Britain is to send military officers to Libya as part of a diplomatic team in the rebel city of Benghazi.
The Foreign Office says they will not train or arm rebel forces, but will give advice on organisational structures, communications and logistics.
Foreign Secretary William Hague said their mission was compatible with the UN resolution on Libya, which ruled out foreign military ground action.
He stressed that the officers would not be involved in any fighting and the move was needed to help protect civilians.
The officers will wear civilian clothing, not uniforms, but are likely to carry sidearms.
The BBC reports there are concerns at the implication of the decision to send them.
Former Liberal Democrat foreign affairs spokesman Sir Menzies Campbell warned against becoming bogged down in Libya, in similar fashion to what happened to the United States in Vietnam.
Labour MP David Winnick also criticised the deployment of British officers.
''However much one despises the brutality of the Gaddafi clan which rules Libya, the fact remains that there is a danger of mission creep,'' he said.
'''There is a civil war in Libya and this is a big escalation of Britain's involvement. I don't think there is an appetite in Britain for military intervention.
''Having been engaged in two wars in nine years in Muslim countries, it would be unwise to become involved in a third,'' said Mr Winnick.
France said it opposes the idea of sending troops to break the military stalemate in Libya.
Foreign Minister Alain Juppe says the situation is difficult and confused, and the West has underestimated Colonel Muammar Gaddafi.
The UN Security Council resolution, passed in March, authorised a no-fly zone over Libya.
Gaddafi future could be discussed - minister
The Libyan foreign minister has implied that the future of Muammar Gaddafi as the country's leader could be up for discussion in the event of a ceasefire.
Abdulati al-Obeidi says Britain's announcement that it is sending army officers to advise Libyan rebels will prolong hostilities.
Mr al-Obeidi said the Libyan government was serious about a properly verifiable ceasefire supervised by foreign observers, the
Mr Obeidi proposed the ceasefire could be followed by an interim period of six months to prepare for an election which would be supervised by the United Nations, as proposed by the African Union.
He said the election could cover any issue raised by Libyans, including, he implied, the future of Colonel Gaddafi as leader.