Obese people should not be hectored and lectured to lose weight, the health secretary, Alan Johnson, is to warn today.

In a speech later today, Johnson will say that pressuring fat people to adopt healthier lifestyles is counterproductive.

His speech at the Fabian Society tonight is seen as a response to the call by the Conservative leader, David Cameron, to hold people accountable for their unhealthy lifestyle choices.

The health secretary will say that while the government has "a moral duty" to tackle obesity, that was not a reason to lecture people about their lifestyles.

"Just as the government has a moral duty to tackle poverty and exclusion, so it also has a duty to address obesity," he is to say. "But this is not a licence to hector and lecture people on how they should spend their lives – not least because this simply won't work.

"Research shows us that vilifying the extremely fat doesn't make people change their behaviour. Commentators who point and shout at pictures of the morbidly obese simply fuel the problem."

He will say such tactics would fail to reach those "whose seriously unhealthy lifestyles are not advertised by their waistlines".

Instead what was needed was a more intelligent approach that set out the facts for those at risk.

"If you present the message more intelligently - if you explain to parents that many children, regardless of their size, have dangerous levels of fat in their arteries or around their organs, and this may reduce their life expectancy by up to 11 years - then people respond," the minister will say.

He will say that obesity is the "biggest health challenge" facing the country and call for a national advice campaign to give obese people the facts about the risks their weight poses to their health.

The campaign should involve employers, retailers, the leisure industry, the media, local government and the voluntary sector providing advice and information about obesity to the public.

The campaign would give "every citizen in the country at every stage of their lives to get the encouragement and support they need to be healthy - from what they see on the television, to what they buy in the local supermarket, to the resources at their disposal in the local community, to how they travel to and from work or school, to the information and advice they get from health professionals," said Johnson.

Earlier this month Cameron called for an end to the reluctance to hold people accountable for their unhealthy lifestyles.

He said: "We talk about people being 'at risk of obesity' instead of talking about people who eat too much and take too little exercise. We talk about people being at risk of poverty, or social exclusion: it's as if these things - obesity, alcohol abuse, drug addiction - are purely external events like a plague or bad weather.

"Of course, circumstances - where you are born, your neighbourhood, your school, and the choices your parents make - have a huge impact. But social problems are often the consequence of the choices that people make."