Children aged five to 15 will travel for free on participating local buses in England in August, the chancellor will announce on Thursday.
Rachel Reeves is to reveal a £100m fare-free scheme designed to relieve some of the ongoing cost-of-living pressures.
The Labour government will also suspend tariffs on more than 100 types of products including biscuits, chocolate, and dried fruit and nuts.
Ministers are pressing supermarkets to cut costs for shoppers but will not force them to cap prices on essentials like eggs, bread and milk.
But even a voluntary scheme to cut costs was described by Marks & Spencer's chief executive, Stuart Machin, as "completely preposterous".
"My advice is that the government should reduce some of the tax and regulatory burden and free us up in a very competitive market," he said.
Treasury minister Dan Tomlinson told the BBC it was "right that the government looks across the board at what more we can do - both government levers but also talking to industry about the steps that they can take to support people with the cost of living".
The BBC has approached other political parties for comment on the free bus travel scheme.
Shadow chancellor Mel Stride acknowledged the "value" of the scheme, telling BBC Breakfast that anything that is going to get prices down is a "good thing".
However, he said that capping costs of food items could lead to shortages on supermarket shelves and create "a lot of pressure" through the supply chain.
Stride said that intervening with markets is "not the right way to go" and the government's focus should be on dealing with the "fundamental problems of inflation".
The government estimates it could save a family with two children who make a weekly return trip at £1.50 per fare approximately £27 in August.
Eligible children can take an unlimited number of trips and will not need registration to travel fare-free.
"As the war in Iran pushes prices up at home, my economic plan is the right one," she said.
"I will continue to make the right choices, to protect households and businesses, and build a stronger and more secure Britain."
Reeves also appeared on the social media app TikTok, writing in the caption: "I'm stepping in to make sure kids can still experience the little joys this summer."
The bus scheme, to run from 1 to 31 August, mirrors one trialled in the West of England which covers summer, Easter and Christmas holidays.
"Kids Go Free in the West of England has seen around 1.4 million free journeys," said Helen Godwin, the Labour mayor of the West of England.
"It's brilliant that, after we have invested devolved funding to make a difference that people across the West can see and feel, the government is rolling out Kids Go Free nationally this August!"
Prime Minister Sir Keir Starmer said: "By giving every child free bus travel throughout August and cutting tariffs on everyday food items, we're putting money back into people's pockets and making life that bit easier."
The full list of products on which tariffs will be suspended will be published next week.
Tariffs are import taxes on goods paid by companies that form part of the supermarket supply chain. Grocers will need to pass on the savings from lower taxes for shoppers to see cheaper products.
In another move, a 5p cut on fuel duty will be extended to the end of the year, instead of being phased out in September, the government said on Wednesday.
The reduced tax rate for petrol and diesel was introduced by the previous Conservative government after Russia's full-scale invasion of Ukraine pushed prices up in 2022.
The price of petrol and diesel has risen significantly since the start of the US-Israel war in Iran, which has affected global supplies of oil and liquified natural gas.
"We're backing drivers by extending the freeze on fuel duty," Sir Keir told the House of Commons.
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