A new analysis suggests so-called Islamic State (IS) militants have lost 22% of the territory they held in Syria and Iraq over the past 14 months.
The data was compiled by research company IHS.
It also estimates that IS has lost 40% of its revenue - much of it from oil - after losing control of much of the Turkish-Syrian border.
Security sources have told BBC Newsnight that the flow of UK jihadists going to fight in Syria is also down.
The loss of territory in Syria, coupled with the fact that several high-profile IS leaders have been killed or captured in recent months, is weakening the group's propaganda message, experts say.
IS militants have lost 8% of territory since IHS last compiled its data less than three months ago.
New recruits are still leaving for Syria but at a slower pace. In total, just over 800 people have left the UK to join extremists in Syria, according to sources who have spoken to Newsnight - the vast majority to fight with IS.
Almost 100 have died there. About 350 are back in the UK.
Losing territory on the ground may also be choking off funding.
Columb Strack, who leads the team responsible for producing the IHS Conflict Monitor maps, says losing control of border crossings with Turkey to the north of the IS stronghold of Raqqa has meant it is far harder to raise money by selling oil on the black market.
When IS controlled the border, smugglers would travel south to buy oil and they would be able to take it out in tankers through the Turkish border. Now Syrian Kurds control the border area, exporting the oil is far harder.
There were reports on Monday that a Chechen fighting in Syria, Omar Shishani, a top Islamic State commander died from injuries sustained in a US air strike. However, the IS-linked Amaq news agency on Tuesday cited an unidentified source as denying Shishani was dead.
American Special Forces also reportedly captured one of IS' chemical weapons experts, Sleiman Daoud al-Afari, last month.
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