A senior member of the British government is threatening to resign if Prime Minister Tony Blair joins a U.S.-led war against Iraq without United Nations approval. The threat from International Development Secretary Clare Short comes at a crucial time for Mr. Blair.

The announcement from Clare Short has hit Britain like a political earthquake. The outspoken Ms. Short accused Mr. Blair of being what she described as "deeply reckless" in his unflinching support of President Bush in the showdown with Iraq.

Ms. Short told British radio she will quit if Britain goes to war without U.N. approval. "If there is not U.N. authority for military action or if there is not U.N. authority for the reconstruction of the country, I will not uphold a breach of international law or this undermining of the U.N., and I will resign from the government," she said.

Mr. Blair's office says the threat took the prime minister by surprise, and he was not consulted in advance.

Blair loyalists accuse Ms. Short of undermining his efforts to win U.N. support for a resolution authorizing military force against Iraq.