David Cameron should "show a bit more class" and answer the questions, Harriet Harman insisted as she quizzed the Prime Minister on the EU referendum and Tory childcare promises.
The interim Labour leader said Mr Cameron could not help but "gloat" about his election success, ignoring the questions posed to him at Prime Minister's Questions in the process.
But Mr Cameron hit back, insisting he was the only Prime Minister who had ever been accused of gloating by quoting the leader of the Opposition.
Reform of Prime Minister's Questions is poised to become a hot topic at Westminster after Speaker John Bercow appeared to give his blessing to cross-party talks on changes being held.
Ms Harman opened the weekly bout by urging the Prime Minister to explain why he did not want to allow 16 and 17-year-olds to vote in the European Union referendum, insisting: "This is about the future of our country, they did in the Scottish referendum, it's their future too."
Mr Cameron replied: "Can I thank you and all the Labour MPs who joined us in the division lobbies last night - after five years of opposing a referendum, to see them all trooping through was the biggest mass conversion since that Chinese general baptised his troops with the hosepipe."
The Prime Minister said MPs should vote on votes at 16 in the referendum during scrutiny of the European Union Referendum Bill, but said he and the Conservatives were against it.
Referring to the first part of Mr Cameron's reply, Ms Harman said: "You won the election, you're the Prime Minister. You don't need to do ranting and sneering and gloating.
"You can just answer the question - and frankly you should show a bit more class."
Ms Harman's remarks were met with loud jeering on the Tory benches.
Ms Harman turned her questions to the imposition of "purdah" on the Government during the referendum campaign, insisting the poll "must be fair and be seen to be fair".
She said: "Why are you changing the law to exempt the Government from the rules which are there to ensure the Government doesn't inappropriately use public funds or the Government machine in the short campaign. Will you think again about this?"
Mr Cameron repeated answers given by Europe Minister David Lidington at the end of the second reading debate, insisting the referendum campaign could not obscure the minister's ability to engage with EU summits, European Court judgements or other issues emanating from Brussels.
He added: "The second issue I would raise - and I think this is a bigger issue - is when the negotiation is complete and the Government has a clear view, I don't want us to be neutral on this issue, I want us to speak clearly and frankly on this issue.
"When it came to the Scottish referendum, I actually felt in the last few weeks before the referendum the UK government was often being advised it couldn't take a view on the future of the UK.
"I think that was a ridiculous situation, which is why we have put forward the change to the purdah rules."
Mr Cameron said it would be debated further during committee stage of the Bill.
Ms Harman replied: "It's not a change in the rules, it's a blanket exemption - we must have a legal framework on the face of the Bill, we cannot be left just to rely on ministerial restraint."
The interim Labour leader said she "strongly agreed" with the Electoral Commission that referendum polling day should not feature any other elections.
She urged Mr Cameron to agree a "separate voting day" for the referendum.
The Prime Minister said it was an "important issue of protest and procedure", adding: "My view is the timing of the referendum should be determined by the timing of the renegotiation. When the renegotiation is complete, we set a date for the referendum.
"I don't myself think it should be determined by the timing of other elections."
Mr Cameron said he had been content the referendum on whether Britain should adopt the alternative vote electoral system was held on the same day as other elections, telling MPs: "I think people are capable of making those two decisions."
Ms Harman changed tack to ask about childcare policies launching this autumn, and challenged: "Are you on track to meet that promise?"
Mr Cameron said: "It's an important principle we are introducing that if families spending £10,000 on childcare they should be able to get £2,000 back.
"This is a Government for working people and wants to help people with the cost of childcare. Not only are we doing that, and the Chancellor will set out the timing of the introduction in his Budget, but we are also doubling to 30 hours the number of hours people will get if they have three and four-year-olds."
Ms Harman said: "It doesn't help working people to make promises and then not meet them."
Ms Harman said the Government had to make sure people would actually benefit from increased free childcare and would not just be hit by higher fees.
Mr Cameron said there would be a review of fees paid by Government and added more details about the tax relief policies.
He added: "What this will mean is families under this Government will have far greater choice on childcare, far greater resources on childcare, and they - rather like you who said the other day 'a greater number of people feel relieved we are not in government' - I suspect those parents will feel the same way."
The Prime Minister's quip prompted Ms Harman to renew her direct attack.
She said: "You just can't help yourself but gloat, can you. Go right ahead, gloat, but why shouldn't you just answer the question about childcare?
"We know - and perhaps we can have an answer rather than a gloating session for the next question - we know that often grandparents help out. Most parents say they just couldn't manage without the grandparents.
"But increasingly those grandparents are not retired, they are themselves working. So will you agree to look at how we can help grandparents get flexibility at work like allowing them to share parental leave?"
Mr Cameron replied he was "happy to look at that" and said the Government had "championed" flexible working.
He added: "I'm sorry if you think I'm gloating. It must be the first time someone has ever been accused of gloating while quoting the leader of the Opposition.
"For instance, you said the other day 'people tend to like a leader who they feel is economically competent'.
"I think you have been talking a lot of sense and I am going to be quoting you as often as I possibly can."
After the session, a Labour spokesman said: "If it leads to PMQs without tribal braying, most people would welcome that."