Labour leader Ed Miliband made the attack at Prime Minister's Questions as concern mounts about the vague assurances Pfizer has made about what it would do if it successfully snaps up AstraZeneca.
"I'm really not going to take any lectures from the guy who was negotiating with Pfizer over the heads of the board of AstraZeneca. Pfizer doesn't need a PR man, they've got the Prime Minister," he said.
He used the government's Royal Mail sale, which was reported to have lost the taxpayer £750 million on its first day of stock market flotation due to it being undervalued, in order to attack Cameron's stance on Pfizer's takeover plans and urge him to block the deal on public interest grounds.
“We all know what happened last time he got assurances. He sold off Royal Mail at a knockdown price and the Chancellor’s best man made a killing. He can’t give us a guarantee because the CEO says he wants to ‘conserve the optionality of splitting up the company’.
“Isn’t it obvious he should have a proper test of the public interest and if the deal doesn’t pass, he should block it.”
Cameron responded to Miliband's attacks by criticising Miliband for initially refusing to meet Pfizer CEO Ian Read as he could not fit in the time while out campaigning.
"The only difference between us is how do you get those things. I say get stuck in, negotiate hard, fight for Britain, he says stand up, play politics and put that before the national interest," he told MPs.
The exchange comes after executives from the two pharmaceutical companies had their second day in a row of being grilled by MPs.
Pfizer chief executive Ian Read told the Commons science committee that the proposed merger - which would be the largest foreign takeover in British corporate history - makes sense from a scientific point of view because "we have complementary portfolios of information in cardiovascular, metabolic and oncology".
Read said he expected the combined research budget of the joint firm to be lower than that of the individual companies, but stressed Pfizer's commitment to 20% of research and development posts to be based in the UK.
Tory MP Stephen Metcalfe suggested "you are going to need far more salesmen than you are scientists to make this deal stack up and pay".
Read said he did not know how many scientists would be employed by a combined firm but added: "I would suspect that there will be less scientists than the natural arithmetic combination of the two."
He told MPs: "We will put into the UK 20% of our global R&D headcount. I think that is an unprecedented commitment. What we are saying to you is this combination would make us the largest pharmaceutical company in the world and we are saying that we are tying the headcount in the UK to 20% of our global."
"I expect the combined research budget put together will go down. Some of these savings will come from duplicate budgets, duplicate science that's going on between the companies. Others come from increased efficiencies in what we do."
Read's initial evidence to MPs on the business committee on Tuesday did not help dismiss fears among politicians that Pfizer could cut jobs if the takeover goes through.
Tory MP David Rutley, whose Macclesfield constituency is home to nearly 2,000 AstraZeneca employees, told HuffPostUK that he found Read's evidence "disappointing" due to its vagueness about Pfizer's plans for Astrazeneca.
"We're none the wiser and that is what is disappointing," he said. "Pfizer has a track record of asset stripping across America, so people aren't willing to take their commitments as well and to regard them as solid. There is quite an issue of trust here."
"The mood has shifted against Pfizer," he warned.
Author Margaret Heffernan wrote on HuffPostUK: "Most mergers and acquisitions don't work. With a failure rate anywhere between 50 and 80 percent, they're bad bets usually placed by leaders who've run out of ideas. This one is no different."