A Ministry of Defense spokesperson told CNN on Thursday that the scientists had conducted the tests in British labs on blood and hair samples from victims, collected at Khan Sheikhun in Syria's idlib province, where 89 people were killed in the April 4 gas attack.
"These have tested positive for the nerve agent Sarin, or a Sarin-like substance," the spokesperson said.
The spokesperson did not say how the samples had reached the United Kingdom but explained that Britain chose to carry out its own analysis and was not working on behalf of any other country or body.
The tests were not part of an official investigation launched by the Organization for the Prohibition of Chemical Weapons in the wake of the attack.
The United States, Turkey and other Western states have blamed the regime of Syrian President Bashar al-Assad for the deaths.
Syria and its most powerful ally, Russia, have vehemently denied the accusations, saying that terrorist groups were behind the deaths.
But several experts have said that only the Syrian regime is likely to have stockpiles of Sarin gas.
Chemical weapons experts have also dismissed a Russian version of events that a Syrian airstrikle hit a terrorist group's weapons depot on the ground, causing a release of deadly gas.
The British statement comes a day after Turkey's Health Minister Recep Akdag said he was "certain" that Sarin gas was used in the attack, based on blood and urine samples.
Turkey has conducted autopsies on at least three of the attack victims.
US officials were the first to say they believed Sarin gas was used in the attack, but they offered no concrete evidence. The World Health Organization soon after the attack said that the victims' symptoms strongly suggested the use of Sarin or a similiar substance.
UK Foreign Secretary Boris Johnson disclosed overnight that the analysis had taken place.
"Today, British scientists have completed an analysis of samples obtained from the site of the attack and concluded that Sarin, or a Sarin-like substance, was used. Our assessment, like that of the US, is that it is highly likely the Assad regime was responsible," he said.