More than 6,000 members of Metropolitan Police staff are due to take industrial action in a dispute over pay.
Members of the Public and Commercial Services union (PCS) employed by the Met will walk out after they were refused a £1,250 London allowance, which is paid to police officers.
PCS general secretary Fran Heathcote said: "Our members in the Met Police have had enough of being treated like an afterthought. Their almost 100% vote for strike action sends a strong message."
The force has been approached for comment but previously said pay and allowances for staff and officers were "completely different" and reflected the "fundamental differences in roles, responsibilities and expectations”.
Ms Heathcote added: "Police staff will not tolerate being treated as second-class citizens within their own workplaces and they will take action until they receive the same £1,250 allowance as their colleagues."
In February, more than 300 members of staff walked out in a dispute over hybrid working.
While civilian staff can go on strike, police officers across the UK are currently banned from taking such action under the Police Act 1996. They have been prohibited from doing so since 1919.