Coalition air strikes have helped reduce Islamic State forces in Syria and Iraq to their lowest levels for two years, Defence Secretary Michael Fallon has told Cabinet.
Around 600 fighters with the terror group have been killed over the past month, bringing the total deaths to around 25,000, said Downing Street.
Mr Fallon briefed Cabinet at its regular meeting in Number 10 ahead of a conference later this week of countries contributing to the military effort against IS - also known as Daesh. He also discussed measures being undertaken to prevent IS establishing footholds in other areas, particularly north Africa.
IS has suffered major setbacks over the past months in Syria at the hands of government forces and US-backed Kurdish fighters including the loss of the historic city of Palmyra.
The self-declared caliphate has also suffered financially from coalition strikes on the oil facilities which provide much of its income and has lost territory in Iraq to the forces of the Baghdad government.
Following the Cabinet meeting, Prime Minister David Cameron's official spokeswoman said: "We have now seen more than 25,000 Daesh fighters killed, over 600 in the last month or so. The number of Daesh fighters is estimated to be at its lowest for about two years.
"We are playing a vital role in terms of the coalition air strikes, we are the second largest contributor behind the US.
"We've seen the Iraqi forces managing to clear Daesh out of the town of Hit in recent works.
"We are working with Kurdish and other partners liberating key areas of Syria and also managing to cut off the main route between Raqqa and Mosul."
RAF jets began air strikes against IS in Iraq in September 2014, and the mission was extended to Syria in December 2015.