Four boats carrying 140 migrants, including children, have landed at the RAF base at Akrotiri in Cyprus, the Ministry of Defence says.
It is believed to be the first time during the current Mediterranean migrant crisis that people have arrived on UK sovereign territory.
The base, on the south coast, has been used to launch British air strikes against Islamic State fighters in Iraq.
A number of Iraqi Kurds are believed to have landed there in 1998.
Vessels carrying the migrants were spotted in the early morning hours off RAF Akrotiri, a sprawling military facility on the southern coast of Cyprus used to bomb Islamic State targets in northern Iraq.
“We have not established where they are from yet,” a spokesman for the bases said.
The base is one of two sovereign territories retained by Britain on Cyprus, a colony until 1960.
Despite its proximity to Syria, EU member Cyprus has not seen any of the massive influx of refugees seen by either Italy or Greece, where arrivals have topped 500,000 this year.
Refugees have tended to avoid the island because of its relative geographical isolation from the rest of Europe and difficulties in leaving.