Johnson said a political transition plan that requires President Bashar al-Assad to step down could help stalled peace talks to restart.
Syria's Foreign Ministry said on Thursday that statements by Britain's Foreign Secretary Boris Johnson demonstrate London's part in "aggression" against Syria and showed he did not understand the situation there.
"Statements of British FM Boris Johnson reveal his complete separation from reality and his lack of realisation that the time of the (British colonial) mandate will not come back," it said in a statement carried on state media.
On Wednesday, Johnson said a political transition plan presented in London by the Syrian opposition that requires President Bashar al-Assad to step down could help stalled peace talks to restart.
Syria remains mired in civil war more than five years after an uprising began against Assad, drawing in both regional and global powers, causing a refugee crisis in the Middle East and Europe and inspiring jihadist attacks across the world.
Hundreds of thousands of Syrians have been killed in the fighting and 11 million - half of Syria's pre-war population - displaced but no party in the multi-sided conflict appears poised for an imminent victory.