By Mary Hanbury for Business Insider US

UK grocery giant Ocado was forced to delay or cancel thousands of orders this weekend after a fire broke at one of its London warehouses, the BBC first reported.

The company said in a statement Monday that three of its robots collided.

Thousands of robots zoom along a grid at its automated warehouses, collecting groceries and taking them to picking stations where employees pack orders. According to the BBC, this warehouse handles up to 150 000 orders a week.

As Insider’s Charlie Floyd previously reported, the grid uses an air traffic control system intended to prevent any crashes.

Ocado said the damage from the fire was limited to 1% of the grid, and that no employees were injured, but that 800 staff were evacuated.

The warehouse was closed for several days, affecting thousands of orders. The fire started on Friday, June 16, and deliveries recommenced Monday morning, Ocado said.

Customers took to Twitter to complain about the lack of clarity around delays and cancellations.

A spokesperson for Ocado did not immediately respond to requests for more details.

Customers who live in other parts of the country were also confused as to why their orders had been affected.

On Tuesday, Insider was unable to request a new delivery slot for a London address. Slots showed up for other addresses in the UK, however.