At least 85 British jihadis and fighters involved in Syria's bloody civil war have been killed, according to the Defence Secretary.
Michael Fallon confirmed around 850 people linked to the UK and regarded as a security threat are now believed to have taken part in the conflict.
While just under 50% are thought to have returned to the UK, more than 10% have been killed, Mr Fallon said.
High-profile British deaths have included Mohammed Emwazi, who became known as Jihadi John after he appeared in propaganda videos from terror group Islamic State (IS) in which British and US hostages were murdered. He was killed in a drone strike last year.
Not all of those fighting in Syria have joined IS, which still retains strongholds in Syria despite increased military pressure.
Shadow Foreign Office minister Diana Johnson said the "grim reality" for those lured to join IS, also referred to as Daesh and Isis, had been highlighted by the figures.
But ministers must do more to explain how the 400 or so "homegrown extremists" are being managed following their return to the UK, Ms Johnson insisted amid concerns that available powers are not being fully used.
Cabinet minister Mr Fallon, in a letter to Ms Johnson, said: "Approximately 850 UK-linked individuals of national security concern have travelled to take part in the Syrian conflict.
"Just under half of these have returned and over 10% have been killed.
"This number includes all those of national security concern who have travelled to take part in the conflict, not just those who have affiliated themselves with Daesh."
He added: "Those returning to the UK after training or fighting with groups such as Daesh can expect to be rigorously investigated to ensure that they do not pose a threat to our national security.
"If the authorities believe they have committed criminal offences, they will be prosecuted."
Mr Fallon disclosed the latest update after Ms Johnson pressed the Government to reveal the number of UK men and women who have travelled to support IS in Syria and those who have returned.
Labour's Ms Johnson told the Press Association: "These figures show the grim reality for the UK citizens who have been lured into joining Isis - it's not glamorous, it's not exciting and you are very likely to end up either dead or tortured.
"But there is also a real issue here about the 400-plus fighters who have gone out to Syria and returned.
"These homegrown extremists pose a serious risk.
"Last year the Government introduced new powers to manage these extremists when they return, but the Government keeps refusing to say how often, if at all, these powers have been used.
"The Government needs to explain what they are doing to manage the risk these individuals pose."
The UK Government believes the number of foreign fighters joining IS has fallen to around 200 a month from its peak of 2,000.