A small army of what unions have branded "scab" volunteers will be used by London Underground bosses to help minimise disruption during a planned 24-hour tube strike.
Hundreds of thousands of passengers face massive travel disruption when up to 4,000 station and ticket staff stage a walkout at 6pm on Sunday (8 January).
The industrial action comes after last-ditch talks at Acas, the conciliation service, broke down on Friday afternoon when RMT and TSSA unions failed to resolve a dispute over staffing levels with Transport for London (TfL).
The majority of stations in Zone 1 of the network are expected to be closed, while TfL has promised to deploy 100 extra buses and enhance river services in a bid to avert travel chaos in the UK capital.
TfL says it also plans to use an as-yet unknown number of "travel ambassadors" to help dampen the impact of the strike.
Made up of TfL employees who volunteer to work shifts during strike days, they will help keep stations open and advise passengers on alternative travel routes.
But the move has been slammed by the RMT as unsafe for passengers, with the union also branding them scabs.
"Instead of resolving the issues, Tube bosses have chosen instead to ramp up the rhetoric with threats to mobilise a strike-breaking army of 'ambassadors' with severe consequences for the current safety regime across the Tube network," Mick Cash said on Friday (6 January).
A RMT spokesman went on to tell IBTimes UK: "The use of these volunteers is inherently dangerous. They are not as trained as our members and [TfL] is taking a risk by using them."
It is not the first time TfL has threatened to use volunteers during strikes on the transport network.
In 2014, London Underground bosses were accused of being provocative after vowing to "beat totally unnecessary industrial action" then staged by the RMT by deploying its travel ambassadors.
The then-general secretary of the RMT, Bob Crow, branded them a "scab army" and called their use "dangerous nonsense".
Steve Griffiths, London Underground's chief operating officer, said on the breakdown in talks with the unions: "It is clear that some more staff for stations are needed. We have started to recruit them and will continue to work with the unions to implement the recommendations made in the review.
"We believe that this will help us to provide a better service for our customers and ensure that they continue to feel safe, secure and able to access the right help while using our network. We encourage the trade unions to continue working with us in order to resolve this dispute and deliver the customer service our customers expect."
TfL have been approached for comment on its planned use of travel ambassadors.