The impact of the cost-of-living crisis on children in the UK already living in poverty would in some cases prove “fatal”, the food writer and campaigner Jack Monroe has told MPs, adding: “And that’s not a term that I use lightly.”

Children and disabled people experiencing food insecurity risked being trapped in a “never-ending loop of difficulty”, including chronic health conditions, mental illness and depression, Monroe told the Commons work and pensions select committee.

Monroe described the situation faced by millions of children living in poverty as “already untenable”, having become increasingly so over the last decade, and called for social security benefits to be uprated in line with inflation.

Individuals on the lowest incomes had been hit hardest by increases in the price of everyday food essentials, and the reduced availability of value product lines, she told MPs. A £20 food shop now bought about two-thirds the amount of goods it did a few years ago.

“And that’s not people deciding not to go to the theatre or not have legs of lamb or bottles of champagne; that is people deciding: ‘We won’t eat on Tuesday or Thursday this week’ or ‘we’ll turn the heating off’ or ‘we’ll skip meals’,” she said.

Last month Monroe successfully campaigned for Asda, her local supermarket in Shoeburyness, Essex, to reintroduce value food lines, and drop prices on a number of basic products such as rice and pasta, which had in some cases gone up by more than 100% in a year. Asda reintroduced the items and prices in all its UK stores and online.

Jack Monroe giving evidence to the work and pensions committee on Wednesday.

Asked by the committee chair, Stephen Timms, why she thought supermarkets had withdrawn their value range, Monroe said: “They thought they could get away with it. They did it for a very long time. They probably thought no one would notice, and the people who did notice weren’t the people who tended to be listened to.”

She added: “I’m grateful they have listened but exasperated that it took a late-night rant on Twitter in order for anybody to pay any attention.”

It was wrong that food retailers in effect were deciding whether customers could access sufficient food, she suggested, saying: “The onus on ensuring that people are able to feed themselves adequately and decently and nutritiously should not fall on the price point of pasta in a supermarket.”

Monroe urged the government to increase social security benefits by at least 6% from April, rather than the planned 3.1% rise. “Even 6% is not going to adequately cover the difference in cost of living, plugging the gap for what people haven’t had for so long, but it’s a start.”

She disputed the notion that it would be difficult for the government to come up with an adequate level of benefits. “It’s simply a matter of giving people the dignity and the advocacy to say: ‘This is what I need,’ and give it to them. No one is asking for the moon. People just want to be able to pay their rent and feed their kids.”

Monroe said people on low incomes were cutting down on food to cope with the soaring costs of rent and energy. “In my experience of 10 years on the coalface of anti-poverty work, I can tell you that people are just eating less or skipping meals or having less nutritious food, bulking out on that 45p white rice and 29p pasta in lieu of being able to have fresh fruit and vegetables and nutritionally balanced meals.

“It’s not that food has got cheaper because it certainly hasn’t. It’s that everything else has got more expensive so there is less in the household budget for food.”