David Cameron agreed that the use of chemical weapons in Syria could trigger western intervention, following a telephone conversation with Barack Obama last night.
It is the first time the prime minister has publicly has even toyed with the idea of intervention in a public forum.
The call came days after Obama said the use of chemical or biological weapons would be a "red line" for the US government and change their "calculus" for intervention.
"Both agreed that the use - or threat - of chemical weapons was completely unacceptable and would force them to revisit their approach so far," a Downing Street spokesperson said after a round of telephone calls between Cameron, Obama and French president Francois Hollande.
The conversations also saw increasingly explicit language about bolstering support for rebel forces in the country.
“[Cameron and Hollande] discussed how to build on the non-lethal support recently announced by the UK and agreed that France and the UK would work more closely to identify how they could bolster the opposition and help a potential transitional Syrian government after the inevitable fall of Assad," the spokesperson said.
The increasingly confident statements of support for the Syrian rebels worries some analysts, who warn that western governments remain ignorant of the make-up of the Free Syria army - especially given the complicated cultural, political and ethnic make up of the country.
Recent supports by human rights groups suggest the rebels are guilty of war crimes, including massacres.
In the US, Obama has come under intense criticism for his "red line" comment. Some observers said the remark effectively gave president Bashar al-Assad carte-blanche to use tanks, warplanes and other conventional weaponry.
"We have been very clear to the Assad regime, but also to other players on the ground, that a red line for us is [if] we start seeing a whole bunch of chemical weapons moving around or being utilised," Obama told reporters.
"That would change my calculus."
Others believe the unusually direct comments were an effort by Obama to foreshadow a justification for later action, if US intelligence suggests Assad intends to use chemical weapons.