a spanner in the works
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a spanner in the worksan event, person, or thing that prevents the smooth or successful implementation of a plan; a drawback or impediment.
A variant, found chiefly in North American English, is a monkey wrench in the works , a monkey wrench being a spanner or wrench with adjustable jaws: to throw a spanner (or a monkey wrench ) into the works is to deliberately wreck someone's plans or activities. In his 1974 novel The Monkey Wrench Gang, Edward Abbey used this as a metaphor for systematic industrial sabotage, and monkey-wrenching is now a colloquial term for such activity.
1997 Spectator Pretty well all the newspapers…are now adversarial in tone, conceiving their basic purpose as throwing spanners in the works almost as a matter of principle.