AstraZeneca on Friday rejected a 63 billion pound ($106 billion) bid from Pfizer, but the U.S. firm is expected to pursue its efforts to acquire Britain's second largest pharmaceutical company.
"We need a more substantive assessment of whether this takeover is in the national economic interest before the UK government allows itself to be seen to be supporting it," Labour leader Ed Miliband said in a letter to Prime Minister David Cameron.
While the government has talked with both AstraZeneca and Pfizer, it has said it has no intention of intervening in the deal and considers it a matter for the companies' boards and shareholders.
But a year away from a national election, which polls show Labour would win if the vote were held now, Miliband's comments emphasise the high political, as well as financial, stakes.
Party leaders want to be seen protecting Britain's interests while avoiding nationalistic rhetoric that may spook other future overseas investment.