A group which is opposed to abortion in all circumstances and favours an abstinence-based approach to sex education has been appointed to advise the government on sexual health.

The Life organisation has been invited to join a new sexual health forum set up to replace the Independent Advisory Group on Sexual Health and HIV.

Stuart Cowie, Life's head of education, said: "We are delighted to be invited into the group, representing views that have not always been around on similar tables in the past."

In contrast, the British Pregnancy Advisory Service (BPAS) has been omitted from the forum despite its long-term position on the previous advisory group and 40-year track record in providing pregnancy counselling nationwide.

"We are disappointed and troubled to learn that having initially been invited to the sexual health forum we have been disinvited, particularly now we understand that Life have been offered a seat at the table," said Ann Furedi, chief executive of BPAS. "We find it puzzling that the Department of Health would want a group that is opposed to abortion and provides no sexual health services on its sexual health forum."

Cowie said Life would seek to build "common ground" with other members of the panel. "If we can be involved with other people in reducing [the number of abortions], then that fits with our charitable objectives and I don't think is unpalatable to anyone else, regardless of their position on when life begins."

However, Life's support for greater emphasis on abstinence when it comes to sexual education is likely to be one of a number of areas where it will be on a collision course with other members of forum. For example, Life has been critical of literature about contraception distributed by the sexual health charity, Brook. They will sit alongside each other at the forum. Life claimed that teenagers were not being told that condoms only gave partial protection against some STIs (sexually transmitted infections) and little or no protection against others.

Brook's national director, Simon Blake, said its literature was based on clinical evidence, and linked the provision of such information to underlying figures from abortion statistics released on Tuesday which showed a reduction in teenage conceptions despite an overall rise in the number of abortions. The under-18 abortion rate has reduced from 17.6 per 1,000 women in 2009 to 16.5 per 1,000 women in 2010.

Blake said: "Having made such massive progress, what we have to do is sustain that … and not go back to a time when the young had really poor sexual and relationship education and see a rise in teenage pregnancy rates and sexually transmitted infections as a result."

The new committee has held one meeting but Life was not represented. The invitation to the group by Anne Milton, the public health minister, appears to have caught some forum members by surprise.

It could yet open up another area of disagreement within the coalition. The former Liberal Democrat MP Evan Harris, warned that the organisation's presence could prevent the panel functioning properly. "When you have an organisation campaigning against the law and against current policy on sexual health, which is pro-contraception and about ensuring that abortion is a choice, then the risk is that you prevent the panel being given access to confidential information," he said.

"It can prevent the advisory panel having frank and open discussions because you have a group there that is committed to opposing current policy."

A Department of Health spokesperson said: "To provide balance, it is important that a wide range of interests and views are represented on the forum.

"Marie Stopes International and the British Pregnancy Advisory Service have similar interests. We offered them shared membership but they declined, and after careful consideration we concluded that it was not feasible to invite both organisations."

BPAS asserts that the department withdrew the offer of 'shared membership'.

The forum consists of representatives of the British Association for Sexual Health and HIV; the Faculty of Sexual and Reproductive Health at the Royal College of Obstetricians and Gynaecologists; the Association of Directors of Public Health; the British HIV Association; the Terrence Higgins Trust; Brook; the Family Planning Association; the Sex Education Forum and National Children's Bureau; Marie Stopes International; and Life.

The department said the criteria applied in terms of appointments to the group was that the core membership would be drawn from national level organisations with a remit covering sexual health across England. They also had to be able to demonstrate clear evidence of impact in improving sexual health and must have a sufficient infrastructure to be able to field deputies at a senior level in the organisation.

Life also became a founding member last week of a new Sex and Relationships Council, which was launched in parliament with the endorsement of the education secretary, Michael Gove.

The council, which includes the Christian-run pro-abstinence group the Silver Ring Thing, says it aims to bring the voice of what it describes as "value-based, parent centred" sex and relationship education (SRE) providers to policy discussions on the future of SRE in schools.

A total of 189,574 abortions were performed in 2010 – a 0.3% increase on the previous year, the figures released on Tuesday show.

Marie Stopes described the rise as small but warned that the figures sent a warning for the government's family planning strategy.

"There are three key areas that need to be focused on: education, access and choice," it said, calling for the delivery of "comprehensive and standardised sex and relationship education in all schools".

In its response to the figures, the Family Planning Association said: "Clearly there needs to be a much better relationship and tighter integration between local contraceptive and abortion services. Despite the advances, women still live in a postcode lottery. Where you live dictates how quickly you'll get an abortion. This is unacceptable."

Life, which provides its own pregnancy counselling services and describes itself as non-denominational, reacted to the figures by suggesting that a "cooling off" period before abortions could play a role in reducing the number being performed.

Some secular organisations have been growing increasingly worried that Tory ministers are opening up government to the agendas of faith-based and pro-life groups.

Some of the same groups have already been preparing to capitalise on the government's big society agenda, which would potentially allow them to replace secular groups in terms of providing services.

In Richmond, south-west London, the Catholic Children's Society has taken over the £89,000 contract to provide advice to schoolchildren on matters including contraception and pregnancies. Another Christian-run charity, Care Confidential, is involved in providing crisis pregnancy advice under the auspices of Newham PCT in east London. Care's education arm, Evaluate, was one of the founding members, alongside Life, of the Sex and Relationships Council.

Meanwhile in parliament, the battle lines on abortion are set to be drawn again after cross-party amendments to the health and social care bill were put forward by anti-abortion MPs in a bid to tighten the rules on terminations.

The first amendment, put forward by Nadine Dorries and Frank Field, would establish a new precondition for any women having an abortion to receive advice and counselling from an organisation that does not itself carry out terminations.