Prime Minister David Cameron will travel to Turkey on Tuesday to meet President Tayyip Erdogan to discuss how to stop Britons crossing into Syria and Iraq to join up with Islamic State (IS) fighters via the Turkish border, his spokesman said.

The visit comes after Britain said last month it was facing its greatest ever national security threat, in part because of the risk that Britons who return from fighting with IS could launch attacks on home soil.

“The focus, particularly given current events in Syria and Iraq and in that region, will be CT (counter terrorism) and ISIL (IS),” Cameron’s official spokesman told reporters on Monday.

Many of the around 500 Britons who the authorities believe have travelled to the region have used Turkey as a transit destination. Around half that number are already believed to have returned to Britain.

“Certainly people travelling overseas from the UK to take part in terror activity is very much part of that relationship we have with (Turkey),” Cameron’s spokesman said.

Cameron will meet Turkish Prime Minister Ahmet Davutoglu on arrival in Ankara followed by an evening meeting with Erdogan, the spokesman said.

(Reporting by William James; Editing by Andrew Osborn)