London, Feb 10 (IANS): Worried over their plight in the UK after Brexit, a group of illegal immigrants decided to flee the country in what authorities described as a "very rare and unusual attempt".

French authorities were shocked to discover 33 runaways hiding in a lorry in Calais, France.

The group of 27 men and four women told immigration officials that they "feared how they would be treated after Brexit", the Metro newspaper quoted the Sunday People daily as saying in a report on Sunday.

"They were found in a lorry and told security that they were leaving because they feared how they would be treated post-Brexit," a source said.

"They said they were worried their rights would be curtailed when we are no longer in the EU. Then they were sent back by French authorities in three lots of 11, escorted by police, and taken to immigration centres."

As of Sunday night, 31 members of the group were back in the UK and being held at immigration detention centres, the Metro newspaper said.

Under a reciprocal agreement, France is able to return migrants to the UK if they are facing criminal charges or have already applied for asylum here.

It is believed the group made the dangerous journey after sneaking on board a container truck which then boarded a Danish-owned DFDS ferry from Dover.

Immigration officials at Calais discovered them hiding in the container after stopping the lorry at a checkpoint.