Tony Blair guaranteed yesterday that the number of asylum-seekers arriving in Britain would halve by September, hardening his language on the issue in angry exchanges in the Commons with Iain Duncan Smith, the Tory leader.

The Prime Minister said last week he believed the target could be met, only for David Blunkett, the Home Secretary, to call it "unachievable". But Mr Blair has now swept aside any ambiguity by issuing a promise that could come back to haunt him in seven months' time.

Asked by Mr Duncan Smith if he had issued an aspiration or a pledge, he replied: "It was a firm commitment, of course." The Tory leader retorted that on Friday Mr Blair had said it was a "pledge". On Saturday, the Home Office had said it was impossible and on Monday a Downing Street spokesman had said it was an aspiration.

Mr Duncan Smith said the Government's asylum policy was a "complete shambles" and changed every time Mr Blair gave an interview on the subject. "Instead of getting on television, why don't you just get on with your job?" he said.

Mr Blair replied: "We can have this conversation again in June when the figures for the first quarter come out and we can have it again in November when the September figures come out."

The Tories had been wrong last year over Labour's vow on street crime, he said. "I think you will be eating your words again over asylum too."