Pfizer Inc. has decided to kill its planned $150 billion takeover of Allergan PLC, after the Obama administration took aim at a deal that would have moved the biggest drug company in the U.S. to Ireland to lower its taxes, according to people familiar with the matter.

The companies are expected to announce the deal’s termination as early as Wednesday morning, after Pfizer’s board voted Tuesday to halt the combination and the New York-based pharmaceutical company then notified Dublin-based Allergan, the people said.

Pfizer will pay Allergan a small breakup fee, the people said. The merger agreement called for a fee covering Allergan’s integration-planning expenses of up to $400 million in the event that tax-rule changes rendered the deal unworkable.

Late Tuesday, the sides were still working out how much the planning had cost Allergan, but it is expected to be much less than the full $400 million, one of the people said.

The breakup fee is relatively small, especially given Pfizer’s market value of some $200 billion.