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There is "no silver bullet" in efforts to change public behaviour, peers examining the theory of "nudging" and its use by ministers have been told.

A top official at the Department of Health told the Lords Science Committee persuading people to quit smoking or lose weight would always be difficult.

Government risked being seen as too intrusive or soft on such issues.

There were signs anti-obesity drives were working but it could take up to 40 years to tell their success, she added.

Before gaining power, leading Conservatives expressed interest in the "nudge" theory of bringing about social change.

Proposed by US academics, this approach argues that bad choices and laziness are a large part of what makes people human and instead of appealing to voters' self-interest, politicians must help to make personal and socially beneficial choices "easier".

Ministers have established a "behavioural insight" team in the Cabinet Office to co-ordinate work in this area and peers are scrutinising its effectiveness and successful cases of behavioural change in a series of public hearings.

The head of communications at the Department of Health said there were signs that parents of children under the age of 11 had changed their buying habits following public information campaigns on healthy eating and exercise, purchasing more fruit and vegetables and less cake.

But Sian Jarvis said it would take "30 to 40 years" to properly evaluate whether there had been a long-term breakthrough in reducing child obesity.

"It is difficult for people to change their behaviour even when they want to," she said.

Government's role was to "make it easier for people to make choices," she told peers and not to come across as a "nanny state".

It was "presentationally difficult to strike the right balance between protecting individual freedoms but also encouraging personal responsibility".

While warnings about the dangers of smoking through advertising had been important in bringing about change, she acknowledged that legislation had had a key role to play as well.

"There are not any silver bullets," she said. "While nudge is very useful, there are a whole range of interventions necessary to create behaviour change."

Gemma Harper, chief social scientist at the Department of Environment, Food and Rural Affairs said behavioural change efforts were regarded by the coalition government as "shorthand for an alternative to regulation or fiscal measures because it is more cost effective".

Asked to provide examples of cross-departmental working on this issue, peers were told the Department of Health and Department of Work and Pensions were collaborating to change "fatalistic attitudes to public health" among communities with high levels of social deprivation.

Better health could provide "spin-offs in other areas such as reduced dependence on incapacity benefit," Dr Sunjai Gupta, head of public health strategy at the Department of Health, said.

But peers expressed concern that any government shift in policy could be classified as behaviourally driven and that any change could not happen without wide public acceptance.

"Government intervention will only be successful if there is a societal climate which is already ready to accept and bring pressure to bear on friends and neighbours," Baroness Perry said.