Prime Minister David Cameron has made a robust defence of the government\'s controversial plans to shake-up the NHS in England. He told Tory activists he did not mind taking \"a bit of a hit\" on the issue. The need for change was \"unavoidable and urgent,\" he said, and claimed the NHS was in \"the party\'s DNA - and that\'s not going to change\". He also told the party\'s spring conference he wanted to make Britain \"stronger and fairer\". Under the NHS plans, GPs and other clinicians will be given much more responsibility for spending the health service budget in England, while greater competition with the private sector will be encouraged. Mr Cameron said it would have been easier not to address an \"invisible crisis\" in the National Health Service in England. \"We could have just protected the NHS from the cuts, as we have, we could have just put in the extra £12.5bn, as we have, and we could have just left it there. \"That would have been easy, but it would have been wrong, because sooner or later the cracks would have started to show; the queues would have grown, patients would have been let down. So, frankly, I don\'t care about taking a bit of a hit on this issue.\" We came in to change the country we love in many ways, we came in to save the country we love” David Cameron Mr Cameron said the government was building on work that had been started by the Labour Party \"because they, too, knew that we had to modernise our NHS\". Various bodies representing specialists across the NHS in England have expressed a range of concerns about the government\'s Health Bill outlining the changes. Among them, the Royal College of GPs - with 42,000 members - wants the bill killed, the Royal College of Physicians of London has concerns about it, while the Royal College of Surgeons of England is generally supportive but with a few concerns. Mr Cameron also told the party members gathered at the private event in central London that \"fortune favours brave governments\". \'Caring\' Conservatives \"Tough and bold action\" would make the UK \"stronger and fairer\", not by playing it safe,\" he said. And he said the government cuts were being made because Conservatives \"care\". \"We didn\'t campaign for 13 long years just to get into government then stick it on cruise control. \"We came in to change the country we love in many ways, we came in to save the country we love. \"And there is only one way of doing that, taking tough action, putting country first, striving to the last.\" BBC political correspondent Carole Walker says Mr Cameron also took on critics of his welfare plans and work experience programmes, saying there must be no right for those who can work to choose to live off the state instead. She says Mr Cameron was defending decisions he knew had been far from popular, telling activists \"when you\'ve inherited the largest deficit in the country\'s peacetime history you can\'t just pick off the low hanging fruit and prune back a little here or a little there. You\'ve got to look at the big items\". Mayoral backing Mr Cameron also urged party members to help Boris Johnson win another term as the mayor of London, describing him as \"a brilliant mayor of the best city on earth\". Mr Johnson later told the conference that voters in London would have to choose between modernisation or a return to the \"irresponsible and unaffordable approach of 1970s Labour\". Former Mayor of London Ken Livingston will be challenging Mr Johnson in the mayoral election on 3 May. \"Do you want to let Livingstone in and take London backwards?\" Mr Johnson asked. \"Or do you want to go forwards and another four years of sensible, moderate, no-nonsense, Conservative administration in London?\" Mr Johnson detailed a nine-point plan for London that included council tax freezes, job creation, putting more police on the beat and cutting Tube delays. \"I feel like a guy who has built half a bridge,\" he said. \"I can see the other side. I can see what needs to be done.\" Mr Livingstone has pledged to cut transport fares by 7% if he is elected. He has also said he would reinstate the Education Maintenance Allowance in the capital.