Holding stand-alone elections for police and crime commissioners will cost an extra £25 million, David Cameron said today as he was accused of backing "reckless and needless" public sector reforms.
Voting for the new post will take place in November next year instead of being held at the same time as local elections in May.
Labour leader Ed Miliband said it was a waste of money and also attacked the Government's NHS shake-up.
In the first Prime Minister's Questions session after the summer recess he called on Mr Cameron to "scrap both of these disruptive and dangerous plans".
The Prime Minister accused Labour of performing a u-turn over support for elected police commissioners and said the party opposed all reform of the NHS.
Mr Cameron said the £25 million additional cost of the elections for commissioners would not come out of police budgets.
But Mr Miliband said: "You are making a bad policy worse by wasting money."
Mr Cameron insisted: "It is important to get this policy right and to make sure it works."
He asked: "Why is the party opposite so frightened of elections? What have they got to fear?"
Mr Cameron claimed shadow police minister Vernon Coaker backed the policy of direct elections to make police more accountable.
But Mr Miliband said: "We know what the public up and down this country know: this is the wrong priority for the country.
"What did we see during the riots? We saw visible, effective policing."
He continued: "The Prime Minister tells us we cannot afford the current police budget, we have got to cut the number of police officers by 16,000.
"But he tells the country it can afford £100 million and more as a result of his decision to waste money on 42 elected politicians earning over £120,000 a year.
"That could pay for 2,000 extra police officers.
"Isn't the truth this is the wrong priority at the wrong time for the country."
But Mr Cameron said the plan involved scrapping police authorities, which would save money.
He added: "Why are you frightened of direct elections so the police become accountable?"
Labour had previously had plans for "directly elected representatives to give local people more control over policing", the Prime Minister said.
He asked Mr Miliband: "Why the U-turn?"
As Mr Miliband moved on to the controversial health reforms, set to clear the Commons later, Mr Cameron joked: "I'm not surprised you want to change subjects because on policing you were having your collar felt because you have done a complete U-turn on the policy you used to be committed to."
The Labour leader said the number of people who had to wait more than six months for an operation had gone up by more than 60% since Mr Cameron entered No 10 Downing Street.
Mr Cameron said: "In our health service what we are seeing is more cancer patients get treatment, more doctors in our NHS, fewer bureaucrats, a reduction in mixed sex (wards) ..."
The Prime Minister was cut off as Speaker John Bercow intervened to rebuke hecklers.
Mr Cameron said: "They don't like hearing good news about what's happening in the National Health Service.
"The fact is, if you look at waiting times for outpatients, they've actually fallen since the last election."
Mr Miliband said there were "people up and down this country who have been waiting longer for their operations".
He said: "The country and I are just asking for a simple explanation from the Prime Minister: why has it happened?"
Mr Cameron insisted: "The amount of time people are waiting for an outpatient operation has actually gone down, that's what's happened.
"As you know, we have targets for 90% of people to get their treatment within 18 weeks and those targets are being met."
The Prime Minister said his health reforms were now backed by the Royal College of GPs, the Royal College of Physicians, the Royal College of Nurses and former Labour health minister and surgeon Lord Darzi.
He added: "Labour have got themselves in a position of opposing all reform to the NHS and opposing the extra money into the NHS."
Mr Miliband told the Prime Minister: "I think you have convinced the country that you are on another planet from them."
He said in newspapers this week the British Medical Association, the Royal College of GPs and the Royal College of Midwives "all rejected your Bill".
Mr Miliband said: "The truth is, under this Government we are seeing two reckless and needless reorganisations of our public services, police numbers down and waiting lists up.
"Under Labour we saw police officers up and waiting lists down.
"Why don't you do the right thing for the future of our public services and scrap both of these disruptive and dangerous plans?"
Following the publication of former chancellor Alistair Darling's memoir, Mr Cameron attacked Mr Miliband, saying: "Isn't it interesting that he doesn't dare, in six questions, mention the economy?"