London: More than 700 Britons are thought to have travelled to Syria and over a half have since returned home where they now pose a significant terrorism threat, British police said on Thursday.

Mark Rowley, the lead officer on counter-terrorism, also said there had been a record 338 arrests for terrorism-related offences last year, up by a third from 2013, with almost a half related to the conflict in Syria.

Jihadi John ... The infamous masked, black-clad militant seen in Islamic State propaganda videos was identified by the Washington Post as a Briton named Mohammed Emwazi. Credit:Reuters

Rowley said of the 700 Britons who were believed to have gone, "a significant proportion" were thought to have tried to join Islamic State (IS) militants who have taken over swathes of Syria and Iraq.

About a half of those who had gone to the region were now thought to be back in Britain.