David Cameron declared yesterday that some people who are poor, fat or addicted to alcohol or drugs have only themselves to blame.

He said that society had been too sensitive in failing to judge the behaviour of others as good or bad, right or wrong, and that it was time for him to speak out against “moral neutrality”.

In a conscious shift of strategy, the Tory leader said he would not shirk from discussing public morality and claimed that social problems were often the consequence of individuals’ choices. “We talk about people being ‘at risk of obesity’ instead of talking about people who eat too much and take too little exercise,” he said. “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 people make.”

The Conservatives retain a commanding lead over Labour — 13 points — according to the latest Times poll.Mr Cameron travelled to Glasgow — a city that he said had inspired his party’s crusade for social justice — to make his boldest appeal yet on restoring personal responsibility.

Ending an era in which politicians have fought shy of judging personal behaviour, he blamed such sensitivity for eroding responsibility over decades.

Mr Cameron attacked the notion that public figures should refuse to use concepts such as right and wrong and, signalling a harder edge to Tory policy making in the months to come, declared he would criticise people who brought misfortune on themselves.

He sought to pre-empt comparisons with John Major’s “back-to-basics” speech, when the former Prime Minister called for a return to traditional values but was portrayed as embarking on a disastrous moral crusade. Speaking of politicians, Mr Cameron said: “Our relationships crack up, our marriages break down, we fail as parents and as citizens just like everyone else. But if the result of this is a stultifying silence about things that really matter, we redouble the failure.”

Aides said that Mr Cameron’s speech was a deliberate toughening of his stance at a time of acute concern over issues such as knife crime. The Tory leader was seeking a mandate to take tough action against those making the wrong choices, they said.

The Conservatives have already unveiled a package of radical welfare reform to strip the workshy of benefits. Mr Cameron backed up his rhetoric yesterday with a commitment to jail anyone convicted of a knife crime.

Some charities expressed concern at the prospect of returning to a more judgmental society. Anne Longfield, chief executive of 4Children, said: “It is always positive to talk around individuals taking responsibility for their actions as long as we accept there also has to be support to help them. Berating individuals because they are in a situation through no fault of their own is not helpful.”

Hugh Thornbury, strategic director of the children’s charity NCH, said: “It’s fine for politicians to judge, but one does need to see the behaviour he’s talking about.”

In the latest Times poll, published today, the Conservatives have suffered a fall over the past month, but retain a commanding lead. They are now on 41 per cent, down four points. Only one of the last 21 published polls, going back to April, has put them below 40 per cent. Labour has gained three points since early June to 28 per cent.

But Mr Cameron enjoys a substantial lead over Gordon Brown in the personal ratings of being strong, a winner and being up to the job of prime minister. Similarly, the Tories have increased their advantage as the best party on key issues, such as managing the economy, crime and taxation.

Mr Cameron’s speech came during a campaign visit to Glasgow East, where Labour is defending its thirdsafest seat in Scotland in a difficult by-election this month.

Labour spent the first weekend of the fight without a candidate after the Glasgow councillor lined up to contest the seat withdrew. Senior party figures were forced to embark on a desperate search for an alternative after George Ryan pulled out.

Margaret Curran, a former minister in the Scottish Parliament, was finally persuaded to stand and last night won the official backing of the local party at a selection meeting.

However, she will face questions at Labour’s campaign launch, amid suggestions that she plans to stay on as an MSP until 2010 even if she is elected to Westminster.

She is likely to be forced to fend off accusations of hypocrisy after she criticised Alex Salmond for retaining his Westminster seat until the next general election despite being elected to Holyrood last year.

The First Minister of Scotland predicted a “political earthquake” in the Glasgow East by-election. The SNP leader made his forecast as he launched his party’s campaign. The Nationalists will need a 22 per cent swing to overturn Labour’s majority of 13,507.

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