Officers were called to the site in Slough, Berkshire, after staff discovered the stowaways in the back of a 37-tonne tanker.

It had been delivering chocolate ingredients to the confectionery firm which produces bars such as Mars, Snickers and Twix.

The 15 men, who were all aged between 20 and 40, were examined by ambulance staff before being arrested, Thames Valley Police said.

They were were "physically fine", a spokesman said.

Three other men on a second lorry which was not going to the factory were also detained.

A spokesman for the UK Border Agency confirmed that 18 men in total were arrested and those found at the Mars site were "covered in cocoa powder".

A Mars spokesperson said: "We can confirm that a number of people were found hiding in a tanker operated by a third party haulier that had arrived at our factory in Slough to deliver raw materials.

"We immediately informed the Thames Valley police who are now investigating the matter.

"In view of the police involvement, you will understand that we cannot comment further."