A £372m government strategy aimed at cutting obesity in England was today dismissed as a toothless set of gimmicks by opposition MPs and health experts.

The strategy, launched today by the health secretary, Alan Johnson, proposes offering cash incentives to obese people to encourage them to lose weight and suggests schools monitor children's lunchboxes to ensure they are healthy.

A healthy food code of good practice is also to be developed by the government, in conjunction with the food and drink industry, with the aim of developing a single, simple method of labelling foods to help people eat well.

But the Conservatives and Liberal Democrats said the strategy did not go far enough to ensure that the food industry played its part in tackling rising rates of obesity.

They criticised the government for failing to enforce a single system of food labelling on retailers and manufacturers, warning this meant shoppers would still be confused by a range of different nutritional information on food packaging.

There has a been a stand-off between the traffic light food labelling system, devised by the government's Food Standards Agency, and systems showing the percentages of guideline daily amounts a product contains.

Tesco, the UK's largest supermarket chain, said today it would reject the traffic light labels even if an independent review of food labelling, announced in the anti-obesity strategy, concluded that a different system was better.

But Johnson signalled a single system could be imposed on the food industry if there was no consensus.

He said: "If we get to the stage where voluntary means seems to be futile and we are not making progress through voluntary means, then we will consider whether we should move to regulation."

The Conservative shadow health secretary, Andrew Lansley, said: "Alan Johnson is still dithering over a proper system of food labelling. People don't choose to be obese, but they do choose what they eat day to day and need information to understand the effect of these choices. The government has a duty to provide this information but hasn't. It has missed an opportunity to make a real difference."

The strategy, Healthy Weight, Healthy Lives, includes a £75m anti-obesity media campaign to help tackle the problem of expanding waistlines and indolence among children.

This includes a healthy lunchbox policy for schools with the aim of ensuring parents do not give their children junk food now banned from school dinners.

A spokesman for the Department for Children, Schools and Families said schools would work out for themselves how to enforce the code. Some may have staff checking lunches, while others would have children monitoring their lunchbox on a "voluntary basis".

But the Association of School and College Leaders (ASCL) said the plan was too extreme.

Its general secretary, Dr John Dunford, said: "In response to being asked to produce a 'healthy lunchbox policy', school leaders will ask, 'Whatever next?'. Will Ofsted inspectors spot a packet of crisps being eaten from a packed lunch and declare the school to be failing in its duty?

"Schools are already serving healthy meals and no longer sell unhealthy snacks and drinks. They emphasise a healthy lifestyle across the curriculum. But they cannot and will not police the contents of lunchboxes. If that is the expectation, it goes a step too far."

The strategy does set out ways to improve adults' lifestyles with the possibility of offering them cash or vouchers to encourage them to exercise and eat healthily.

But the King's Fund, the healthcare thinktank, warned that such bribes might not prove effective in changing people's behaviour.

Its acting director of policy, Anna Dixon, said: "Health programmes that rely on financial incentives as a lever to promote healthier behaviour are attractive as they are simple and easy to implement.

"But so far they have been most effective where the tasks are simple and time limited. They are less effective where the behaviour change required is complex, as is the case with obesity. Programmes to assist people change their unhealthy behaviours must support long-term maintenance of weight loss, not just short-term successes."