Pfizer and Pharmacia employ a total of 1,700 people in the Republic, with the bulk of that number engaged in manufacturing in the Cork region.

A spokesman for Pfizer declined yesterday to speculate on how the merger would affect Irish employees, saying that the two companies had only reached "day one" in a process that would last a number of months.

"We know that it is going to be top of mind between now and the close," he added.

An IDA Ireland spokesman said that past experience of mergers of a comparable scale suggested some reason for optimism, particularly as pharmaceutical plants tended to be difficult to integrate.

"The record in the past has been good for Ireland," he said.

"In all mergers, Ireland tends to be the gainer."

The authority was in close contact with both companies, the spokesman said.

Pfizer is by far the biggest employer of the two companies, employing 1,300 staff at its manufacturing plants in the Ringaskiddy area of Cork, and a further 200 in sterile and confectionery operations in Dublin.

Pharmacia, meanwhile, employs 220 people at its bulk pharmaceutical plant at Little Island, also in Cork.