The Isis fighters advancing across Iraq are also plotting British attacks, Prime Minister David Cameron said yesterday as he warned it would be a grave mistake to ignore the threat to Britain posed by Islamist extremists in the Middle East.
About 400 Britons are believed to have joined the Isis forces in Syria and ministers believe many will inevitably have crossed the border and be among the militants who have seized wide stretches of northern Iraq.
The Government released figures showing a sharp rise in numbers of "Syria-related arrests" of British nationals and UK residents in recent months. Of the 65 arrests since the beginning of 2013, 40 were made in the first three months of this year.
The security services' biggest fear is that some extremists could bring back to Britain the deadly techniques they acquired in Syria and Iraq.
Several people have been stopped from travelling to conflict zones and the Government is pledging a "sharp focus" on the danger from jihadists returning from the two countries.
It also plans new legislation to prosecute people who plan and train for terrorism abroad.
Cameron told MPs: "I disagree with those people who think this is nothing to do with us and that if they want to have some sort of extreme Islamist regime in the middle of Iraq, it won't affect us. It will.
"The people in that regime, as well as trying to take territory, are also planning to attack us here at home in the United Kingdom."