throw a spanner in the works

• throw a wrench into the works (US)
• throw a monkey wrench in the works

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throw a spanner in the works (third-person singular simple present throws a spanner in the works, present participle throwing a spanner in the works, simple past threw a spanner in the works, past participle thrown a spanner in the works)

1. (idiomatic, Britain) To introduce a problem, dilemma or obstacle, something unexpected or troublesome.

   Halfway through the production of Macbeth, the director found that the stage was smaller than he had expected. This really threw a spanner in the works.

• put a spoke in someone's wheel
• monkey wrench (US)
• spanner in the works