Parliament's cleaners could upset the order of the house this summer by going on strike in a pay dispute.
Strike action could take place during the next five weeks
The Transport and General Workers Union is to ballot 170 cleaners on action as they battle for a pay rise from £5 to £6.70 an hour and extra holidays.
The cleaners were being "screwed" by parliamentary authorities, Jack Dromey of the TGWU said. He said 250 MPs and peers supported the workers.
A Commons spokeswoman said a "mutually acceptable solution" was being sought.
She said the authorities were working closely with the employers and the union to sort out the dispute.
But Mr Dromey, deputy general secretary of the union, singled out Parliament's handling of cleaning contracts for criticism.
He said: "The problem is Parliament. Parliament screwed the contractors and as a result our members are getting screwed.
"Parliament must now face up to its responsibilities. That means varying the contracts, putting more money into the contracts so that the two contractors can pay the London living wage."
He said the members "wanted a decent standard of living in what is Europe's most expensive capital city".
He said any industrial action was likely to take place before Parliament's summer recess, scheduled for 21 July.