a spanner in the works an 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.

2011Mark HeuringMr Dilettante On balance, however, he's been much more of a hindrance than a help to his successors. It's time he stopped throwing spanners in the works.