Boris Johnson was accused of "governing in hindsight" over a series of U-turns, as he appeared before MPs at PMQs for the first time since July.
Labour leader Sir Keir Starmer claimed the prime minister was "making it up as he goes along".
And he said even Mr Johnson's own MPs had "run out of patience" after what he claimed was 12 U-turns over the summer.
The PM hit back by calling Sir Keir "captain hindsight" over the exam results debacle.
He accused the Labour leader of "leaping on a bandwagon, opposing a policy that he supported two weeks ago".
The SNP's Westminster leader Ian Blackford claimed Mr Johnson had made eight U-turns this year - and he called for a ninth to extend the government's job retention scheme, which ends next month, echoing a call made by Sir Keir.
The PM insisted "indefinite furlough" was not the answer to help the economy through the pandemic.
With grumblings on the Tory benches about the government's recent performance Boris Johnson needed a good PMQs to mark the return to parliament.
His political opponents - perhaps unsurprisingly - criticised the number of policy U-turns in recent months.
While ministers have repeatedly said they're responding to changing science as the pandemic progresses, the speed and frequency of policy shifts is the crux of concern among some Conservative backbenchers.
Keir Starmer returned to what some supporters have called a "forensic" style of questioning in pushing the prime minister for detail on the exam results crisis.
Boris Johnson responded with a wide-ranging attack on the Labour leader which led to a tetchy exchange.
But with another shift in policy - this time on local lockdowns in Trafford and Bolton - taking place as the prime minister was at the dispatch box, it seems unlikely his performance was enough to silence critics - including those within his own party.
In heated exchanges, Sir Keir told the PM: "This has been a wasted summer. The government should have spent it preparing for the autumn and winter.
"Instead, they have lurched from crisis to crisis, U-turn to U-turn."
He accused the government of "serial incompetence", and asked: "Will the prime minister take responsibility and finally get a grip?"
Mr Johnson hit back by citing a series of alleged U-turns made by Sir Keir in the past and - in a reference to his predecessor as Labour leader Jeremy Corbyn - accusing him of supporting "an IRA-condoning politician who wanted to get out of Nato".
Speaker Sir Lindsey Hoyle intervened to warn the prime minster "to answer the questions that have been put" to him.
A clearly angry Sir Keir said: "As Director of Public Prosecutions, I prosecuted serious terrorists for five years, working with the intelligence and security forces and with the police in Northern Ireland.
"I ask the prime minister to have the decency to withdraw that comment."
Speaking afterwards, Labour sources said they would not be taking the matter further, but added that the PM had supported a peerage for former Brexit Party MEP, Claire Fox, who had once been a member of a far left party which defended an IRA attack,