The US-backed Syrian Democratic Forces (SDF) say they have liberated the last area held by so-called Islamic State in the eastern village of Baghouz.
The SDF announcement claimed victory over IS and the end of their self-declared caliphate, but they pledged to continue the fight until all remnants of the group are eradicated.
The UK and US, along with other coalition partners, have been supporting the fight against IS for the last four years.
Following the announcement, the Prime Minister Theresa May said: "The liberation of the last Daesh-held territory wouldn't have been possible without the immense courage of UK military and our allies."
"We will continue to do what is necessary to protect the British people, our Allies and partners from the threat Daesh poses."
Defence Secretary Gavin Williamson said: "This has been a long campaign ending the misery of millions, as Daesh swept through Syria and Iraq, and came so close to taking the city of Baghdad.
"But due to the tireless efforts of our service personnel, we have been able to beat them back, depriving them of territory and making sure that Britain is safer."
“But we cannot be complacent. They’ve dispersed, and they’ll continue to pose a threat to Britain, and that is why we will always remain vigilant."
The battle to capture Baghouz lasted for weeks amid an unexpected exodus of civilians from the area.
In the last two weeks, many IS fighters appeared to be among those leaving.
Major General Christopher Ghika, Deputy Commander of the Global Coalition’s Combined Joint Task Force, told Forces News: "This is an organisation which set its stall in 2014 in holding physical terrain, today they hold nothing and therefore it’s a significant moment.
"It isn’t the end of the group or the campaign because IS still present a threat to Iraq, Syria, the region and the world, and that’s why the global coalition will continue its operations.
"They have now been driven underground but they are still credible, they have sleeper cells in Iraq and Syria and that’s why the coalition’s operations will continue.
"It’s absolutely not over. The operation isn’t over because ISIS are still present even though discredited."
In December US President Donald Trump said IS had been defeated and promised to withdraw US troops from Syria.
American troops will remain in Syria, although President Trump mentioned on several occasions that he expects all territory held by IS to be reclaimed.
Around 400 US troops are expected to stay, with half of them deployed on a peacekeeping mission.
The UK is the second largest contributor in the coalition against IS, behind the US, with British jets flying missions from RAF Akrotiri in Cyprus.
Operation Shader began in September 2014 with air strikes over Iraq and extended to Syria the following year.
RAF air strikes in Iraq and Syria killed and injured an estimated 4,315 enemies with just one civilian fatality, according to estimated figures released by the Ministry of Defence.
Maj Gen Ghika says Op Shader will still continue, despite the defeat of IS: "I think the British contribution to Operation Shader, which is principally in the air component, will continue but also the very important work to advise and train the Iraqi security forces which we’ve been doing since 2014.
"That will continue because the Iraqi state still needs capable security forces to counter IS."