British lawmakers are set to vote Tuesday on lowering the time limit for women to terminate a pregnancy, in what would be the first major change to abortion laws here in nearly 20 years.

The evening vote comes after the House of Commons backed by a clear margin Monday the creation of hybrid animal-human embryos, which can supply stem cells for medical research into conditions like Alzheimer's and Parkinson's.

Lawmakers also rejected a bid to outlaw "saviour siblings" -- children created as a genetic match for a sick brother or sister so their genetic material can treat them.

All the issues are being debated as part of the Human Embryology and Fertilisation Bill.

But the vote on whether the current 24 week abortion limit should be cut to 22, 20 or 16 weeks is expected to be much closer.

A first vote was expected around 7:00 pm (1800 GMT), followed by a related ballot around 10:00 pm (2100 GMT).

A few hours before the vote, campaigners handed in a 60,000-signature petition calling for a cut in the time-limit for abortions. Prime Minister Gordon Brown has said that he will vote to retain the existing limit.

The wide-ranging bill would also give lesbians easier access to in vitro fertilisation (IVF) treatment by removing requirements for clinics to consider a child's need for a father in measures also being debated and voted Tuesday.

Britain has one of the highest abortion rates, and latest cut-off dates for the procedure, in the European Union. In 2006, 193,700 abortions were performed on women living in England and Wales, according to official figures.

Abortion was legalised in 1967.

Supporters of lowering the abortion time limit say that survival rates of premature babies have increased with better medical technology and that it should therefore be changed.

Nadine Dorries, a lawmaker for the main opposition Conservatives who is proposing a 20-week limit, told the Daily Mail newspaper earlier this month: "We have reached the point where we need to pull back on abortion.

"If we don't there is no question that we will overtake America in the next couple of years, making us the abortion capital of the world."

But the British Medical Association (BMA), the professional body for doctors here, does not support a reduction in the current time limit, along with several other bodies representing medics.

"It is the BMA's view... that there is no evidence of significant improvement in the survival of extremely pre-term infants to support reducing the 24-week limit for legal abortion," it said in a briefing paper ahead of the debate.

Some 86 lawmakers have signed a parliamentary motion backing the 24-week limit and saying the best way to cut abortions is by reducing unwanted pregnancies through better sex education and more access to contraception.

Parliamentarians have a free vote on sensitive parts of the bill, meaning that they do not have to follow a party line and can follow their consciences.

The bill must be approved by unelected upper chamber the House of Lords before it can pass into law.