A number of boats carrying dozens of migrants, including children, have landed at the RAF base at Akrotiri in Cyprus, the Ministry of Defence says.
It is thought there were about 140 people on board, but their country of origin has not yet been established.
It is believed to be the first time during the current Mediterranean migrant crisis that people have arrived on UK sovereign territory.
The MoD said responsibility for them rested with the Cypriot authorities.
The base at Akrotiri, on the south coast of the island, has been used to launch British air strikes against Islamic State fighters in Iraq.
An MoD spokesman said: "We can confirm that a number of boats with migrants on board have landed on the shore of RAF Akrotiri in Cyprus.
"At the moment our key priority is ensuring everybody on board is safe and well and when we can release further details we will.
"We have had an agreement in place with the Republic of Cyprus since 2003 to ensure that the Cypriot authorities take responsibility in circumstances like this."
The boats arrived early on Wednesday morning, the MoD said.
"Events like this underline why it is important for us to develop a comprehensive approach to the migration crisis working with our international partners to provide humanitarian assistance in Syria and neighbouring countries; to disrupt the trafficking gangs and to address the root causes of instability that cause people to seek a new life elsewhere," the spokesman added.
Almost 600,000 migrants are estimated to have arrived in Europe by sea so far this year, with the majority coming from Syria, Afghanistan and Eritrea.
Unlike a number of Greek or Italian islands, European Union member Cyprus has not seen an influx, despite its relative proximity - less than 100 miles - to the Syrian coast.
EU Commission President Jean-Claude Juncker has called for an extraordinary summit of several EU and Balkan leaders on Sunday to discuss the migrant crisis.
In a statement on Wednesday, he said there was "a need for much greater co-operation, more extensive consultation and immediate operational action".
A number of Iraqi Kurds landed at RAF Akrotiri in 1998 and still live in a second British base on Cyprus, Dhekelia, in former military accommodation.
They have tried to apply for asylum in the UK, but have been repeatedly turned down by the government.