Filming is taking place at The Galleria outlet shopping centre in Hatfield.
The car park between the Odeon cinema and the main shopping mall has been taken over as a production base for the crew, with lorries and trailers parked there.
A crew was spotted setting up in the empty former Frankie & Benny's restaurant opposite Burger King today (Friday).
Producers have put up 'Caution - filming in progress' notices in the walkway beneath the Odeon cinema, between McDonald's and Burger King.
The signs state: "The producers and crew would like to apologise for any inconvenience caused during the preparation / filming of this production.
"We appreciate your cooperation and thank you for your patience."
It is not known at this time what the filming project is.
The Galleria has been used before for films and TV shoots.
The shopping centre featured prominently in 2001 movie The Martins starring Lee Evans and Kathy Burke.
More as we get it.
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