Four men charged with slavery offences following a police raid on a Travellers' caravan site were due in court Tuesday over the treatment of four alleged victims.

The accused, all from the same family, were to appear before Luton Magistrates' Court charged with conspiring to holding a person in servitude and requiring them to perform forced labour.

They are Tommy Connors, 30, Patrick Connors, 19, James (Big Jim) Connors, 34, and James (Jimmy) Connors, 23, all from the Green Acres caravan site outside Leighton Buzzard in Bedfordshire, northwest of London.

More than 200 police officers entered the caravan site Sunday, removing 24 men thought to have been held against their will, including a number of foreign nationals from eastern Europe.

"These charges relate to four victims who allege they have been held against their will and forced to live and work like slaves," said Adrian Roberts of the Crown Prosecution Service.

Police were continuing to investigate other possible offences relating to other potential victims, he added.

Besides the four men, one woman was also arrested. She is heavily pregnant and was released on bail Monday. She will be questioned once she has given birth, Bedfordshire Police said.

Sunday's operation followed months of investigation and detectives said they were looking for two other suspects.

Of the 24, men removed, nine have left the medical reception centre and chosen not to support the police investigation.

"The remaining 15 continue to be assessed for medical and welfare needs," Bedfordshire Police said in a statement.

"It will take a number of days to establish exactly what has happened to them while living on the site."

Those remaining include three Britons, three Poles, one Latvian, one Lithuanian and two men whose nationalities are yet to be confirmed, said police.

The oldest is aged 50 and the youngest is 30.

Of the men who chose to leave, seven were British and two were Romanian. The youngest was a 17-year-old Briton who has re-joined his family.

"Those people who we continue to help are appreciative of the support that is on offer, but it will take some time to work through with them what has happened," said Detective Chief Inspector Sean O'Neil.