Professor Vivette Glover from Queen Charlotte's and Chelsea Hospital in London, says she believes that foetuses over 17 weeks old may feel pain.
She says doctors should consider carrying out terminations under anaesthetic.
The majority of abortions carried out in the UK are done in the first 13 weeks of pregnancy - but some are performed at a later stage.
Professor Glover's view contradicts a report issued by the Royal College of Obstetrics and Gynaecology (RCOG) which said there was no possibility of foetal awareness before 26 weeks.
She said it was currently impossible to determine whether a foetus feels pain at an earlier stage.
But she said study of the development of the foetal nervous system suggested that they did.
'Benefit of the doubt'
She told the BBC: "I think that the evidence is that the system is starting to form by 20 weeks, maybe by 17 weeks.
"Given there is a possibility (that a foetus can feel pain) we should give the foetus the benefit of the doubt."
Professor Glover said it was possible that foetuses did not feel pain in the same way that people did after birth.
She said it could be much more simple response, and it was unlikely to produce the feelings of anxiety common in people.
But she said that obstetricians should be aware of the possibility, and should consider giving anaesthetic directly to the foetus when carrying out an abortion.
Prolife response
Kevin Male, a spokesman for the Prolife charity Life, said other doctors had echoed Professor Glover's findings.
He told BBC News Online: "This is more evidence tat human life exists from the moment of conception.
"We have known it all along, and I suspect that everybody else knows it subconsciously, but will not admit it.
"Abortion is proof that we live in a society with an element of barbarism in it."
However, Ann Furedi, director of communications for the British Pregnancy Advisory Service, the UK's largest independent provider of abortions, said no new evidence had been presented to suggest the foetus could feel pain.
She said: "The expert advice presented to the Department of Health, in 1995,when it sought advice in response to similar reports, was that before 26 weeks the brain is insufficiently developed to 'feel' or 'experience' pain. This advice is still supported by the RCOG.
"Even Professor Glover acknowledges that 'it is incredibly unlikely that the foetus could feel anything before 13 weeks'.
"At BPAS, although the well-being of the woman is our primary concern, we take all possible measures to treat the foetus with respect and care, and we would not knowingly expose it to unnecessary suffering.'
The RCOG position paper, written by Professor Maria Fitzgerald, of University College London, says that "little sensory input" reaches the brain of the developing foetus before 26 weeks.
"Therefore reactions to noxious stimuli cannot be interpreted as feeling or perceiving pain."