Thirteen migrants, including a child, were found huddled at the back of a lorry at Heathrow Airport.
They were intercepted outside a cargo terminal near the west London airport on Saturday.
Police did not say where the migrants were from, but said one person was arrested in connection with the incident.
A Metropolitan Police spokesman confirmed that the force was called to Shoreham Road East, Hounslow, at 1.30pm on Sunday.
"A member of the public had called police to a number of people in a lorry container", a statement said.
"The container was opened and 12 adults and one child were found inside in a distressed condition. None of them are believed to have life-threatening injuries.
"A lorry driver has been arrested. Inquires continue."
The Home Office told The National that it was investigating the incident.
“The government is tackling illegal immigration and the harm it causes, often to the most vulnerable people, by removing those with no right to be in the UK," it said.
“We continue to work with law enforcement agencies to tackle illegal migration in all its forms. Our New Plan for Immigration will speed up the removal of those who have entered the UK illegally.”
Britain has been dealing with an influx of migrants in recent months. Figures released by the government showed that about 20,000 people reached its shores by boat this year.
That figure is more than double the 8,000 who made the crossing across the English Channel and other sea routes in 2020.
The Home Office said outdated Border Force vessels will be replaced by modern boats as part of a £700 million effort to improve policing of Britain’s borders, the Treasury said on Sunday.
The announcement was made before Chancellor Rishi Sunak’s budget and included £628m “to modernise and digitalise the border”, with proposals including a US-style Electronic Travel Authorisation for tourists wishing to travel to the UK.