LONDON — Britain on Saturday threatened Iran with “serious consequences” for seizing a British-owned oil tanker the previous evening as the government warned ships to avoid the crucial shipping lanes of the Strait of Hormuz.

The British government said in a statement after an emergency meeting that it had “advised U.K. shipping to stay out of the area for an interim period.”

The crisis has caught Britain at a singularly vulnerable moment. Prime Minister Theresa May is expected to resign on Wednesday. A leadership contest within the governing Conservative Party to determine her successor has all but paralyzed the government. And now the uncertainty about Britain’s internal direction is compounding the problem of forming a response to Iran’s seizure of the tanker.

The British defense minister, Penny Mordaunt, said in a television interview on Saturday that the ship had been intercepted in Omani, not Iranian, waters and called the seizure “a hostile act.” By Saturday afternoon, Britain had summoned the Iranian ambassador to register its protest, and a second emergency cabinet meeting was set to begin.