Deputy Prime Minister Nick Clegg has spoken out against proposals to offer pregnant women shopping vouchers to give up smoking.

Researchers at the universities of Glasgow and Stirling found that 23% of mothers-to-be offered £400 in online shopping vouchers to stop smoking during their pregnancy succeeded in quitting, compared to just 9% taking part in a normal NHS programme.

The researchers suggested the initiative could be a "potentially cost effective new intervention" to cut health problems linked to smoking during pregnancy, which the study said could include up to 5,000 miscarriages a year, along with hundreds of stillbirths and infant deaths.

The annual cost to the NHS was up to £64 million for treating smoking-related problems in mothers and up to £23.5 million for the babies up to 12 months old, they said.

But Mr Clegg told LBC radio he was "perturbed" at the idea that the NHS could be asked effectively to "bribe" mothers to look after their babies' health.

"Where does this end?" asked the Liberal Democrat leader. "Do we then give people cash in hand to provide their children with a healthy regular meal? Do we give people cheques to make sure their kids go to bed on time?

"At the end of the day, in any free, open, liberal society, yes there are things we have got to try to encourage people to do, but we have got to assume that people take responsibility for their own life and for that of their loved ones.

"It is not for the state to come up with ever more exotic ways of incentivising - or worse trying to bribe - people to do what is good for themselves and their loved ones.

"It's an interesting study ... (but) I am perturbed. I don't think it's the right way forward."