The warning came after a telephone call between Prime Minister David Cameron and US President Barack Obama.
The PM also spoke to French President Francois Hollande. The three discussed building support for the opposition.
Earlier, Chinese state media accused Mr Obama of using the chemical arms issue as an excuse for military intervention.
Also on Wednesday, fierce fighting raged in the northern Syrian city of Aleppo and in Damascus, which residents said had witnessed the heaviest attack by government forces since the army re-asserted its control of the capital last month.
A Downing Street spokesman said the "appalling situation that continues in Syria" was the main focus of Mr Cameron's conversations with Mr Hollande and Mr Obama.
Mr Cameron and Mr Obama both agreed "that the use - or threat - of chemical weapons was completely unacceptable and would force them to revisit their approach so far", said the spokesman.
The comments echoed those by Mr Obama earlier in the week, when he said he would change his thinking on intervention if Syria used chemical weapons.
The two leaders, along with Mr Hollande, discussed "how to build on the support already given to the opposition" and "help a potential transitional Syrian government after the inevitable fall of [President Bashar al-] Assad".
Mr Obama and Mr Cameron called for a "credible opposition" that would "show real unity of purpose and coherence in working towards transition".
The three leaders also discussed the plight of Syrian refugees.
The spokesman said: "The prime minister emphasised the need to work with the UN and... that more should be done by the international community to channel humanitarian aid through the UN appeal."
Earlier, the Chinese state news agency Xinhua accused Western powers of "digging deep for excuses to intervene militarily".
In its commentary, Xinhua criticised Mr Obama's earlier remarks as "dangerously irresponsible" and said they would aggravate the conflict, reducing the chances of a political settlement.
China insists a ceasefire and UN-led mediation remain the best ways to end Syria's woes.
China and Russia have both blocked attempts to impose UN sanctions on Syria.
A Russian foreign ministry source told the Kommersant newspaper on Wednesday that Moscow believed Syria had no intention of using its chemical weapons and was able to safeguard them.
Fierce fighting continued across Syria on Wednesday.
An aerial bombardment preceded an assault by tanks on several areas of Damascus.
Activists said at least 37 people had been killed in the capital, in the areas of Kafar Soussa and Nahr Eishah.
A journalist working for the state-run Tishreen newspaper, Mosaab al-Odallah, was killed by the military during house-to-house searches in Nahr Eishah, activists and friends said.
Odallah was said to be sympathetic to the opposition.
Reuters reporters said they had heard shells and gunfire every minute in the northern city of Aleppo.
Elsewhere, rebels and troops fought for control of a military base and airfield near the eastern town of Albu Kamal.
Activists said at least three people were killed in a helicopter bombardment of Qastoun, in Hama province.
Shelling was also reported in Deraa, and heavy fighting was reported in Deir Ezzor in the east.
The opposition Syrian Observatory for Human Rights said at least 115 people, including 71 civilians, were killed across the country on Wednesday.
The figures cannot be independently verified.
Opposition activists say more than 20,000 people - mostly civilians - have died since the uprising against Mr Assad began last year.
OP: Play nice and kill as many men women and children as you like with conventional weapons and we won't fucking lift a finger. As long as you don't use nasty chemical weapons to blow people's bodies apart you can fill your boots, boys!