Pfizer, the US pharmaceutical giant, will this week step up its charm offensive with AstraZeneca's investors as it tries to seal a £63bn tie-up.
The company is hoping talks with UK-based institutions will encourage its rival to enter into discussions. Major shareholders such as Axa Investment Managers and Schroders have indicated they would like to see both parties start talks.
Sources close to Pfizer say the group is reluctant to increase its £50-a-share offer, with AstraZeneca holding out for something closer to £55 a share before it comes to the table. However, it is hoping that investors will force AstraZeneca into action. The news came as Labour leader Ed Miliband called for an inquiry into the controversial bid and accused the Government of "cheerleading" for a deal.
Pfizer stands to save up to $20bn (£11.8bn) a year in taxes if the deal goes through.
Mr Miliband said: "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," he said in a letter to Prime Minister David Cameron.
In response, Grant Shapps, the Conservative Party chairman, said: "We are going to have tests which ensure that this get-together becomes a great Anglo-American project, or it doesn't happen."