The government announced at the weekend it is sending two British warships to join other Royal Navy vessels already off the coast of Lebanon in preparation for a mass evacuation.
In an interview with British Broadcasting Corp. radio, ambassador James Watt said around 40 British citizens who "needed to move" had left Sunday on board a helicopter, with another party of undisclosed size leaving by helicopter Monday.
A Foreign Office spokesman said the first people to leave were children, the elderly and the ill. It was not immediately clear where they were taken.
The two warships - HMS Illustrious and HMS Bulwark - were dispatched to the area as Israel's land, sea and air offensive against Lebanon continues, but officials say they are not expected to arrive before Wednesday, the Ap reports.
The Foreign Office spokesman said a British military reconnaissance team has arrived in Beirut "to carry out detailed planning for a possible evacuation."
He said the Foreign Office has made contingency plans to bring evacuees to a reception center on the Mediterranean island of Cyprus.