Britain said on Sunday that evidence of a chemical weapons attack in the suburbs of Damascus could have already been destroyed ahead of a visit to the site by UN inspectors.
Earlier on Monday Syria agreed to let experts from the United Nations on Monday visit the site of the reported poisonous gas attack which killed hundreds of people on Wednesday of last week.
"We have to be realistic now about what the UN team can achieve," Foreign Secretary William Hague told reporters.
"The fact is that much of the evidence could have been destroyed by that artillery bombardment. Other evidence could have degraded over the last few days and other evidence could have been tampered with," he said, referring to opposition activists' reports that the army has shelled the area in the last few days.
Hague said that all evidence pointed towards the use of chemical weapons by the Syrian government, and that such attacks demanded a strong international response.
"We cannot in the 21st century allow the idea that chemical weapons can be used with impunity," Hague said. "We believe it's very important that there is a strong response and that dictators ... know that the use of chemical weapons is to cross a line and that the world will respond when that line is crossed."