Travellers moved on by policemen from the PC World car park in Ealing have left large amounts of rubbish behind, according to officers.
An Ealing Metropolitan Police officer said the travellers had "left a fair bit of mess" after uprooting from the site.
The group of around 20 caravans and two trucks arrived in the area on February 10 and stayed for six days before police obtained an order to move them on.
It is the second time travellers have moved onto land in Northolt in little over six months.
Caravans have previously pitched up at the shut-down car park, in White Hart Roundabout, Ruislip Road in July.
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