A supermarket was on lockdown last night after a car believed to have been involved in an armed robbery was spotted in the car park.

The car was seen travelling through north Watford and later found next to Asda, near the Dome roundabout, at around 10pm.

When armed police arrived at the scene, two men ran away.

Members of the public reported being blocked in the supermarket and the car park while police searched the area.

A police helicopter was also called.

Officers arrested two further people on suspicion of robbery – a 25-year-old woman from Borehamwood, and a 22-year-old woman from London.

Both remain in custody.

The car is believed to have been involved in an armed robbery at a Tesco Express in High Wycombe, which happened at 9.20pm the same evening.