Britons have been warned to leave Lebanon as soon as possible amid escalating hostilities between Israel and Hezbollah.
An estimated 10,000 British nationals are reported to be in the country.
Around 700 UK troops will move to nearby Cyprus as the government continues to prepare its contingency plans following “significant escalation” of action between Israel and Lebanon in recent days.
More than 500 people were killed in Lebanon in 24 hours.
UK military teams will be supported by Border Force and FCDO officials.
A statement said: “The government continues to advise against all travel to Lebanon, as the situation continues to deteriorate rapidly, with devastating consequences.”
The RAF has aircraft and transport helicopters on standby “to provide support if necessary” if an evacuation is required.
Commercial flights continue to operate from Beirut airport but with cancellations.
Airlines serving the Lebanese capital with flights to Europe include Middle East Airlines (MEA) to London and Air France to Paris.
Other carriers continue to operate to destinations including Cyprus, Doha, Dubai and Istanbul.
Defence secretary John Healey said: “Events in the past hours and days have demonstrated how volatile this situation is, which is why our message is clear, British nationals should leave now.
“We continue to urge all sides to step back from conflict to prevent further tragic loss of life.
“Our government is ensuring all preparations are in place to support British nationals should the situation deteriorate.”