The Conservative leader yesterday urged the government to tighten security checks on asylum seekers.

His call followed news that some of those arrested over the ricin poison plot were refugees.

Iain Duncan Smith told BBC radio that "we have to be careful. about who we let in".

"I think it shows to what degree we have to be vigilant about people who are likely to try to undermine our society and commit terrorist acts."

The government said tough measures were already in place.

"We do security checks on people, we are not complacent," said a Home Office official.