The vast bulk of foreign prisoners should be deported whatever the dangers in their home nations, says Tony Blair.
Mr Blair's comments were later clarified by No 10
Mr Blair told MPs he was prepared to change the law to ensure most foreign prisoners were deported automatically.
Downing Street later said some prisoners could avoid deportation in "very few exceptional cases" such as a known threat to an individual.
The Conservatives said Mr Blair was "rattled" while the Lib Dems argued the policy was probably unlawful.
Mr Blair's comments follow the news that 1,023 foreign prisoners were released without being considered for deportation.
That cost former Home Secretary Charles Clarke his job and his successor, John Reid, this week revealed that 98 serious offenders, including one murderer were still at large.
It later emerged that more than 1,500 criminals sentenced in the 12 months up to March were recommended by the courts to be deported at the end of their sentence.
Constitutional affairs minister Harriet Harman revealed the figure when responding to a written question.
Earlier in the Commons, Mr Blair came under pressure from Tory leader David Cameron about his promise to deport automatically all foreigners convicted of an imprisonable offence.
The prime minister said the measure might not apply to foreigners who had been in the UK for a long time and only served a short jail term.
"In the vast bulk of cases, as was explained, there will be an automatic presumption now to deport - and the vast bulk of those people will be deported," he said.
"Those people, in my view, should be deported irrespective of any claim that they have that the country to which they are going back may not be safe.
"That is why it is important, if necessary, that we look at legislating to ensure that such an automatic presumption applies."
They should not be able to claim their home country was too unsafe generally, he said, pointing out that people were now being deported to Iraq and Afghanistan.
The prime minister's official spokesman later clarified that a general threat to safety would not be sufficient to avoid deportation but a specific threat could be enough in some cases.
Former Labour Home Secretary David Blunkett said the vast bulk of foreign prisoners could be deported by using a list of safe countries.
But the government would face difficult challenges in the European Court of Human Rights.
Former Home Office adviser Nick Pearce, who is now director of the Institute for Public Policy Research, said the Human Rights Act should not be confused with making moral choices.
"You do actually have to decide whether you want to put somebody on a plane to be tortured or killed," he said.
"That is a harsh judgement to make."
Sir Andrew Green, from Migrationwatch UK, said he too was "surprised" by Mr Blair's words but said he thought the current balance was wrong.
"Too much weight is being given to the possibility of someone suffering when they get home and too little weight is given to risk to British society," he said.
Earlier, Mr Blair came under fire when he confirmed there were no official estimates of the number of illegal immigrants.
He said asylum applications were down and the numbers of failed asylum seekers being deported had increased since 1999.
And the introduction of "electronic borders" planned for 2007 and the planned identity card scheme would help control immigration, he added.