Ministers engaged with bosses from British-based AstraZeneca about its potential takeover by Pfizer before they talked to executives from the US drug giant, David Cameron has said.
The Prime Minister denied the Government had engaged more closely with the American firm over its mooted £60 billion-plus buyout of the UK pharmaceutical company.
Amid fears that post-takeover cost-cutting could cause the loss of thousands of highly-skilled jobs and undermine the UK's science base, Mr Cameron insisted commitments already made by Pfizer should not be underestimated.
Asked on BBC One's Andrew Marr Show if the Government had been engaging more closely with Pfizer on the deal, the PM said: "That is not the case, ministers were talking to AstraZeneca before anyone spoke to Pfizer, so that is not the case.
"Don't underestimate the power of some of the things they have already said, about making sure, for instance, 20% of their R&D jobs will be here in the UK.
"I want more but it's very interesting isn't it that the Labour leader's reaction on being asked to engage with Pfizer is to say no, I'm too busy, because he wanted to stand on the sidelines and make a series of point-scoring political points rather than engaging in the national interest."
Deputy Prime Minister Nick Clegg stressed the importance of getting binding commitments from Pfizer.
He said bitter past experience showed the need for concrete pledges from Pfizer.
Mr Clegg told the Sky News Murnaghan programme: "I think bitter experience in other contexts has taught us about the importance of really pinning these kind of commitments down and not just allowing them to rest as aspirations but making sure that they are exacting, binding commitments.
"There are various ways that that could be made more solid if you like, and that's exactly the kind of thing that we're discussing with Pfizer if they choose to go ahead with the bid."
Meanwhile, Labour's shadow business secretary Chuka Umunna called on the Government to strengthen the public interest test for the takeover.
Writing to Business Secretary Vince Cable, he said powers can be invoked without the need to pass any new laws so any potential buyout would be subject to an assessment of how it impacted on Britain's strategic research and development and science base.
Mr Umunna wrote: " Under the provisions of the Enterprise Act 2002 you have the power to add further categories to the current public interest test, expanding the circumstances in which the Government can issue a public interest intervention notice.
"This does not require primary legislation. You could therefore include a category, as we have suggested, that covers the maintenance of our strategic R&D and science base.
"You could implement this now and therefore enable this potential transaction to be assessed against this. We would support you in adding this category and want to work with you in a cross-party way to make this happen.
"As I have said there is a strong willingness to work with you to enable a stronger public interest test made a reality. Therefore I would welcome the opportunity to meet with you at the earliest opportunity to discuss this further."
Andrew Miller, chair of the Commons Science and Technology Committee, who will quiz both Pfizer and AstraZeneca bosses about the potential deal, said the US company needs to give "very, very" long-term commitments.
Looking at Pfizer's track record, Mr Miller raised concerns the company would cut research expenditure, adding that ten-year or longer pledges are needed for pharmaceutical research to bear fruit.
He told BBC Radio 4's The World This Weekend: "The evidence shows that when they took over Wyeth, an American-owned company, the research expenditure reduced substantially.
"We need to see commitments about that research expenditure, the fact that it's going to happen in the UK and that it happens over a very long time.
"Because a molecule that's discovered today will not be an active ingredient in a medicine that you or I take for a minimum of ten years.
"Given that the time-frames of development in pharmaceuticals we need very, very long-term commitments."