Tesco has apologised to its online customers unable to get on its website as the supermarket seeks to cope with high demand for Christmas bookings.

The UK's largest retailer has had to install a queuing system online to help it to manage the demand.

Some customers complained to Tesco that they had been waiting hours to get onto the supermarket's website.

"We're sorry if things take a bit longer than usual," Tesco said on its Twitter account.

"A lot of customers are using our website and app at the moment."

Tesco said it was "using a virtual waiting room to help us manage the flow".

Supermarkets have been overwhelmed with demand as people start to plan for Christmas, and rival Ocado has already sold out of slots after "huge" demand.

One Tesco customer said on Twitter: "We use delivery saver because we have 3 young children and both parents work full time - I'm currently stuck in your queue while trying to get the children ready for school before I have to work full time to book my usual weekly slot. I've been staring at that screen for an hour!"

Tesco said that by late morning the waiting room had been removed and slots should be available again.

A Tesco spokesperson said: "Demand for online slots over the festive period is high, and we have more slots this Christmas than ever before.

"We experienced high volumes of traffic to our website and groceries app this morning and temporarily limited the number of customers using it.

"We've now removed the waiting room and customers will be able to log straight on. We're sorry for any inconvenience this caused and would like to reassure customers that there are still slots available for home delivery and Click+Collect over the Christmas period."