The Church of England is urging staff and volunteers at homeless night shelters to be on the lookout for signs of exploitation among guests this winter.
The warning comes from the Church of England's anti-slavery campaign, The Clewer Initiative, which is distributing over a thousand posters and other materials to winter night shelters to raise awareness of modern day slavery.
Possible signs of exploitation include "unusual anxiety" towards people in positions of authority or an "extreme fear of being watched".
Other signs may include "having no control of their ID", evidence of physical abuse or untreated injuries, or working for little or no pay, particularly if they work in the construction or car wash industries.
Night shelter staff are also being encouraged to keep an eye out for signs of guests being approached outside the building by someone offering work.
In addition to a poster, the Let's Talk initiative provides guidance notes for project managers and volunteers in night shelters about the steps they can take to safeguard their guests from danger.
The Rt Rev Dr Alastair Redfern, Chair of The Clewer Initiative, said: "Time and time again in our work around the country we meet volunteers and project leaders who have encountered modern slavery and have either not recognised it, or not known what to do about it.
"With rising numbers of homeless people on our streets, it is even more important that we are able to recognise the signs. With the Let's Talk resources we will equip the Church to understand what modern slavery looks like, and how they can respond to protect the vulnerable."
Anna Heydon, Development Worker for Imagine Norfolk Together, the joint venture between Church Urban Fund and the Diocese of Norwich, said: "Modern slavery is a clear and present danger to homeless people in Norfolk and across the country.
"Churches come into contact with so many vulnerable people and I am encouraged to see more and more people understanding how they can help protect them from modern slavery.
"The Let's Talk Resources from The Clewer Initiative will help to give a clear message to guests and volunteers that though homeless people are in danger, there is support to help them."