Waitrose has come in for more fierce criticism over its online delivery site, after it failed to deliver a series of planned updates to fix problems that have plagued the site since it launched four months ago.

Waitrose released another apologetic statement on its official 'Our Website' online forum, confirming a delay to the "next round of improvements", which had been planned for the beginning of this month. The statement said the improvements would go live at the end of July instead.

But it could only confirm it was "working on" changes to previous ­orders and shopping lists, navigation, the search function and the checkout. It did not offer a completion date.

Furious customers have filled Waitrose's forum since the new website was unveiled in March. The first poster to complain blasted it as "terrible" and "so slow" and also said if the website was "like this next week I will consider going elsewhere".

But 16 weeks later complaints are still being made on a daily basis. This week a post read: "diabolical layout, slow, extremely poor in searching items I used to purchase, products display and categorisation are beyond frustration!!"

Another user, who also claimed the website was "slower than ever" agreed the search function failed to operate properly after his search for a bottle of TCP returned a bag of baby carrots. Other users announced that they had switched to Ocado.

Waitrose told The Grocer it was making progress. "Our latest round of changes went live on 20 July, which included improving the speed, usability and accessibility of the site" said Waitrose e-commerce director Robin Phillips. "We are continuing to make incremental improvements, which is absolutely normal at this stage in a site's development."

The news of fresh problems with the website came as Waitrose finally went head-to-head with Ocado this month after restrictions that prevented it operating an online delivery service within the M25 were lifted.