Supermarket Tesco has said that online shoppers will have their deliveries capped at 80 items from now on.
In an email to customers it said that the aim was "to increase the number of customers we can safely deliver to".
It is the latest move by supermarkets to safeguard people and supplies during the coronavirus pandemic.
Supermarkets will also use a government database of 1.5 million vulnerable shoppers in England to help prioritise delivery slots.
Tesco has already limited all products to a maximum of three per customer, whether they shop online or in stores.
Announcing the latest measure, the retail giant said that online orders already placed were unaffected unless customers make changes to them. Anyone who edits an existing order will become subject to the 80-item limit.
Tesco added: "We're at full capacity for the next few weeks and we ask those who are able to safely come to stores to do so, instead of shopping online.
"This will help us to free up more slots for the more vulnerable."
That view was echoed by Iceland's managing director, Richard Walker, who urged healthy people to disregard government advice and shop in person.
He told the BBC's Today programme that the firm had "done their best" to restrict online orders to those most in need, but any new slots released were "quickly snapped up".
Mr Walker referred to Prime Minister Boris Johnson's call for everyone to use food delivery services wherever possible, saying that demand "vastly" exceeded supply.
He added: "I'd actually urge the opposite of the PM, in that, if you are healthy, not in a vulnerable category and adhere to social distancing guidelines, please do shop in store, but make sure you shop responsibly."
A government database of vulnerable shoppers in England will be used by retailers to help prioritise delivery slots during the coronavirus pandemic.
Sainsbury's and Waitrose say they will begin writing to people on the list next week.
The supermarkets say they are also working hard to secure details for vulnerable people living in Wales, Scotland and Northern Ireland.
There is concern those most in danger from the virus are going in to stores due to a lack of online shopping slots.
People in high-risk households have told the BBC they are struggling to get priority treatment online or in stores.
'I'm terrified of going to the shops'
Sharon Cranfield from Tadworth in Surrey says her 19-year-old daughter Jessica is very vulnerable to coronavirus because she has cystic fibrosis, which means she needs an "athlete-style" diet of 3,000 to 4,000 calories a day to survive.
The government says people should book home deliveries where possible, especially if they share a household with someone who is especially vulnerable to catching the virus.
But Sharon has struggled to book an online slot.
"The biggest worry is the lack of availability of supermarket delivery slots," she told the BBC. "For my daughter, it's very scary, because she's in isolation. I'm terrified of going to the shops - I don't want to pick up anything.
"It's so risky and I can't put that risk on my daughter. I just hope they get those delivery slots sorted.
"There's a misconception about the vulnerable being old. A lot of people with cystic fibrosis are children and they can't look after themselves. It's just so stressful. My anxiety levels have gone through the roof."
'I'm forced to mix with people'
Norman Philips, aged 68, cares for his wife and his 91-year-old mother, who both have dementia, but he is not old enough to be allowed to shop early.
Many supermarkets have started to set aside the first hour of trading on certain days for the over-70s, but this does not extend to those caring for them.
But Mr Philips told the Today programme he was unable to secure a slot for online delivery of his groceries, because the first slot available was two weeks away. Facing a huge surge in demand, online shopping sites have been crashing, temporarily closing, or considerably oversubscribed.
As a result, he had to brave huge queues at his local Sainsbury’s.
“I’m forced to go mixing with people and then come back to a home which has got someone who is supposed to be being shielded and another, my mum, who is in the high-risk category,” he said.
He said he had been told by social services to call the supermarket and get his name on a priority list, but despite waiting on the phone for two-and-a-half hours, he was unable to get through.
Another shopper, Claire from Orpington, contacted Radio 5 live’s Wake Up To Money programme to say that although she is disabled, her needs are being overlooked.
“I personally am blind and have a serious heart condition which, while it doesn't put me on the list in terms of being immune-suppressed etc, does carry a risk of serious complications should I catch the virus,” she said in a text message.
“But apparently, I and others like me don't count as any kind of priority and as such, we either take our chances that a slot may become available, or risk catching the virus with the added complications if we go out.”
Claire added it was difficult for blind people to observe the social distancing guidance of staying two metres away from other people, since they had to be guided round a shop.
The concerns come as supermarkets struggle to cope with increased demand for their online shopping platforms. Asda and Ocado’s websites have virtual queues to get into their booking systems.
Sainsbury’s chief executive Mike Coupe said that from next week, he would write to vulnerable shoppers on the government list who are already registered with the supermarket and offer them delivery slots.
Sainsbury’s says it has already contacted 270,000 vulnerable customers to offer support, but many others have complained that they did not receive an email and could not get through on the phones.
Tesco boss Dave Lewis said customers who could get out and shop should do so, leaving delivery slots available for more vulnerable people.
Meanwhile, Morrisons said it had begun delivering the first of 115,000 promised food parcels to vulnerable people.
So far it has delivered 10,000 of the £30 grocery boxes and plans to increase supplies in the coming week.
Stuart Rose, chief executive of Ocado, defended his firm’s performance in the face on an “exponential” increase in demand.
“If you’ve had customers who’ve been regular customers for 10, 15 years, you can’t just dump those customers,” he said.
“We have put in place restrictions on how much they can order, how often they can order.”
Mr Rose called on shoppers to “show more restraint”, saying: “There is no shortage of food. Nobody will starve.”