The legal time limit for abortions could be re-examined because of new medical research, Tony Blair has said.
A 23-week-old foetus in the womb
The prime minister was asked on Wednesday about calls for the 24-week time limit for terminations for
"social" reasons to be halved.
A new type of ultrasound scan last month produced vivid pictures of a 12 week-old foetus "walking" in the womb.
Mr Blair said it was sensible to watch new findings but No 10 stressed he was not signalling any government law plan.
The prime minister's official spokesman denied Mr Blair had raised expectations by his comments in the House of Commons.
Free vote promise
He told MPs at prime minister's questions: "I have not had an opportunity myself to study in detail the
evidence that has been provided.
"But I am sure that if the situation does change then it would be advisable
for us to have another look at the whole question.
"If the scientific evidence has shifted then it is obviously sensible for us
to take that into account.
"If we have proposals to put before the House we will put them."
Mr Blair promised Parliament a free vote on any proposed changes.
Former Liberal leader Lord Steel, the architect of Britain's abortion laws, has led calls for the upper time limit for terminations to be reduced from 24 weeks to 22 weeks.
He also wants mothers to be able to have abortions on demand in the first three months of pregnancy, as happens in France. In the UK, two doctors have to agree to the termination.
Lord Steel said restricting a mother's right to choose by reducing the 24-week upper limit had to be balanced by the fact that some foetuses had survived after born at 22 weeks into a pregnancy.
"I think people find it very repugnant to think you are getting close to the point where you are not dealing with a foetus but with the possibility of a baby," he told BBC News.
Lord Steel, the retired presiding officer of the Scottish Parliament, introduced the controversial Abortion Bill in 1966.
It became law in 1967 with backing from the then Home Secretary, Roy Jenkins.
The 1967 Act legalised abortion in the UK and set a time limit of 28 weeks.
It was cut to 24 weeks in 1990, with termination only permitted in the case of
Health Secretary John Reid on Sunday said abortion had always been a conscience issue which was debated without government sponsorship.
He told BBC One's Breakfast with Frost: "Roughly once every five or six years there is a debate on this.
"It is not up to me as a minister to decide, this is up to Parliament to
Mr Blair's comments were welcomed by Professor Jack Scarisbrick, national chairman of the anti-abortion group Life.
He said: "It is absolutely certain now, from all the evidence available, that
human life begins at fertilisation. From that moment on there is a real, living
Professor Scarisbrick said Mr Blair was perhaps beginning to grasp that abortion was a human rights issue.