The Government has been accused by campaigners of being "schizophrenic" in its handling of the humanitarian crisis in Syria.
Medical charity Doctors of the World made the comments after being turned away from Downing Street with a giant version of a Christmas card they had hoped to deliver to Theresa May.
The massive festive greeting, which depicts a traditional nativity image in the Middle East with a modern drone flying overhead, was aimed at stirring the Prime Minister into committing support to civilians caught up in the country's bloody civil war.
Their intervention comes as a mass evacuation continues of the formerly besieged eastern area of Aleppo, which has bore the brunt of intense aerial bombardments from the Syrian government and Russian forces in recent months.
Representatives of the charity, which helps provide healthcare to some of the world's most desperate areas, claimed they were told at the gates of Downing Street they should instead try to post the card.
Executive director Leigh Daynes told the Press Association: "On the one hand, the UK Government's response is schizophrenic.
"The UK Government is the second-largest bilateral donor to the humanitarian response to the Syrian crisis, which is a good and welcome thing.
"But on the other hand, the entire international community is paralysed by the lack of political action to solve the Syria crisis once and for all - and that's where the Prime Minister needs to step up."
He called on Mrs May to assert pressure on the UN Security Council for the opening of air corridors to facilitate aid drops to inaccessible parts of Syria and for the deployment of monitors across the country.
The organisation is selling smaller versions of the "Star of Bedlam" cards as part of fundraising drive to provide assistance to those fleeing countries ravaged by wars.
Inside the bespoke version addressed to the Prime Minister, it read: "Dear Theresa May, join the world in seeing the realities in the Middle East this Xmas."
Tens of thousands of civilians and fighters have been bussed out of the former rebel strongholds in Aleppo during the past week, as President Bashar Assad's forces prepared to assume full control of the city for the first time in four years.
Mr Daynes said: "Aleppo is not the only besieged area inside of Syria. There are many other places which are in desperate, dire need which are not reached by the UN and international humanitarian organisations."