NHS guidelines recommending pregnant women be asked to take breath tests to prove if they smoke have been backed by midwives.

The Royal College of Midwives (RCM) had previously criticised the idea because they could make women feel guilty. GPs should encourage women to quit smoking instead, it had said.

An estimated 20% of pregnant women smoke, which is believed to lead to a low birth weight for their babies and cause complications in pregnancy and labour.

The National Institute for Health and Care Excellence (Nice) has recommended that women should be tested for carbon monoxide found in cigarettes during antenatal appointments and be provided with help to quit if levels are too high.

Cathy Warwick, the RCM's chief executive, said on Sunday that the tests were only "a partial solution" but added that they could help midwives educate women in the hope they reduce their baby's exposure to cigarette smoke.

She said: "Midwives have a vital role to play in promoting public health, and reducing smoking in pregnancy is extremely important. I visited a maternity unit this week, and heard from fellow midwives just how helpful these tests can be in showing women the potential damage that smoking can have on their baby.

"Of course, not all women will want to take this test. Any test which becomes routine must be offered along with comprehensive information and women must be able to opt out."

Warwick added that the NHS had too few midwives and argued that the real solution to cutting the prevalence of smoking during pregnancy was more midwives and more continuity of care from the same midwife.Along with arguing that the use of breath tests make women feel guilty, RCM officials had expressed scepticism on the basis of the costs associated with all midwives carrying monitors and issues such as safety and infection control.