Boris Johnson plans to break rail strikes by allowing use of agency workers

Unions condemn measure as potentially against international law as 40,000 staff prepare for first day of industrial action

Boris Johnson has responded to the biggest rail strikes in a generation with plans to break the industrial action by allowing firms to bring in agency staff, a move unions have decried as unworkable, unsafe and potentially breaking international law.

As 40,000 workers prepared for Tuesday's strike, the most wide-reaching on the railways in 30 years, Downing Street brought forward changes to enable employers to replace employees with temporary staff.