The Foreign Secretary has called on Russia and Iran to ensure humanitarian aid can be safely delivered to besieged areas of Syria.
Philip Hammond said both countries should use their influence over the Assad regime to allow aid drops to reach desperate citizens.
His plea came as a deadline set by world powers for President Bashar Assad to grant aid access to 19 blockaded areas passed on Wednesday.
Mr Hammond said the leader's reluctance to comply with aid efforts, allowing convoys into only two areas, meant risky aid drops are now the last solution.
He said: "The International Syria Support Group (ISSG) agreed last month that if humanitarian aid was still being denied to any area by June 1, the UN should launch a programme for emergency deliveries by air.
"On the day of that deadline, the Assad regime has cynically allowed limited amounts of aid into Daraya and Moadamiyeh but it has failed to deliver the widespread humanitarian access called for by the international community.
"While air drops are complex, costly and risky, they are now the last resort to relieve human suffering across many besieged areas.
"Countries with influence over the Assad regime such as Russia and Iran must now ensure that these air operations can proceed in a safe and secure manner."
The Red Cross reported that aid entered the Damascus suburbs of Daraya and Moadamiyeh on Wednesday after the Government agreed to a 48-hour ceasefire.
It was the first time since 2012 that aid has reached Daraya, which has been subject to a crippling government blockade since residents expelled security forces in the early stages of the 2011 uprising.
The US and Russia-led ISSG last month called on the UN to "immediately carry out a programme for air bridges and air drops for all areas in need" starting on June 1 if it was denied access to designated areas.