Cleaners, security staff and other workers on the Docklands Light Railway have launched a 48-hour strike in a dispute over pay.
Members of the Rail, Maritime and Transport union (RMT) employed by ISS on the London railway have rejected an offer the union said was worth 1.8%.
The RMT claimed ISS staff on the DLR are underpaid compared with others on Transport for London performing equivalent roles.
“They already pay staff poorly and with inflation rapidly increasing an offer of 1.8% shows they do not care one jot for their staff.
“RMT will continue their campaign for pay justice for these workers who are some of the most exploited in the transport system.
“Ultimately mayor Sadiq Khan needs to end the injustice of exploitation of contracted-out staff by bringing these workers back in house as soon as possible.”
An ISS spokesman said: “We are disappointed that this action is going ahead despite our pay offer, which is in line with the London living wage and the terms of our contract.
“We value the contribution of every ISS team member. We can also confirm that cleaners on TfL contracts employed by ISS will get free transport across the network and we will be in touch with the RMT about this soon.”