(Updates with new Hague quote, poll, Obama call)
LONDON, Aug 28 (Reuters) - Britain's national security will be undermined if it fails to challenge the Syrian government over the use of chemical weapons against its own people, British Foreign Secretary William Hague said on Wednesday.
In an article published before Prime Minister David Cameron chairs Britain's National Security Council to finalise recommendations for a possible military response against Syria, Hague said the risks of doing nothing were too great.
"We must proceed in a careful and thoughtful way, but we cannot permit our own security to be undermined by the creeping normalisation of the use of weapons that the world has spent decades trying to control and eradicate," Hague wrote in Britain's Daily Telegraph newspaper.
Now was the moment for democratic nations to live up to their values, he added, saying that doing nothing would make further chemical attacks more likely, "increase the risk that these weapons could fall into the wrong hands", and "fatally undermine" global rules prohibiting chemical weapons use.
Hague said it was the first time that chemical warfare has been used anywhere in the world in the 21st century, saying that efforts to bring Syria to book through the United Nations had failed. "We cannot allow diplomatic paralysis to be a shield for the perpetrators of these crimes," he wrote.
Lawmakers will debate Britain's response to the chemical attack in Syria on Thursday after Cameron cut short his holiday, recalled parliament, and rushed back to London to chair a meeting of the National Security Council.
Cameron spoke to U.S. President Barack Obama on the phone on Tuesday night, a spokesman from his office said, saying both men had agreed there was "no doubt" that the Syrian government had been behind last week's chemical attack.
Public support for action is fragile with many lawmakers across the political spectrum uneasy about the prospect of air strikes. A YouGov poll published on Wednesday showed 50 percent of the British public opposed a missile strike on Syria, with just 25 percent in favour of it. (Reporting by Li-mei Hoang and Andrew Osborn, Editing by Guy Faulconbridge)