The group evaded security in Europoort, Rotterdam, and hid on the freight deck of the Pride of Hull ferry.
After it docked, they were found during a routine UK Border Force search. The ferry left the Netherlands at 9pm (Wednesday) and arrived at Hull at 7am Thursday.
The driver of the lorry, a 51-year-old man, has been arrested on suspicion of facilitating illegal immigration and is also in custody pending interview by Immigration Enforcement crime team officers.
It is understood that none of the men and women aboard came into contact with P&O customers.
A spokesman for P&O Ferries said: “We will continue to work closely with the UK Border Force and call on Europoort to strengthen security measures.”
A Home Office spokesman said: “Border Force officers at the port of Hull detained eighteen people on suspicion of entering the UK illegally this morning (Thursday 18 February) following a search of a lorry that had arrived on a ferry from Rotterdam.
“All eighteen people are now in detention pending interview by immigration officers.”