The government has let the asylum system get "out of control", the Conservative leader has warned.

Speaking on Sunday, Iain Duncan Smith called on the government to take extra powers to deport those asylum seekers who are criminals or pose a terrorist threat.

Ministers had been "weak" in dealing with unfounded asylum claims, he said.

Britain should continue to welcome those genuinely fleeing political persecution, Duncan Smith said, but "we are not seeing that at the moment".

"The vast majority who are coming are coming for reasons that are nothing to do with real political persecution - either for economic reasons or as in a smaller number, but a significant number, now for criminal or terrorist reasons," he told Sky News's Sunday with Adam Boulton programme.

The government was now in a "terrible mess" on the asylum issue. "They've been weak when faced with a serious problem," he said.

"Now inside all of that we have got a problem because we now discover that there are terrorists who have actually used the asylum system to come to the UK and, it appears, to operate from here."

Duncan Smith warned that the asylum system was now "out of control".

The prime minister said that the government would take extra powers to deal with new threats if it felt they were needed.

Speaking to the BBC's Breakfast with Frost programme, Tony Blair indicated that ministers would also reconsider the application of the European Convention on HumanRights if they felt it necessary.

He said the government was already cracking down on illegal asylum seekers.

"The key to this is to get the application numbers substantially down. Because the problem with removing people is that, under the obligations we have, you cannot remove someone to a country where they might be subject to torture."