The SNP will cap supermarket prices for essential goods such as bread and milk if it retains power, John Swinney has pledged, after describing the cost of living as “the defining issue of this election”.

With polls pointing to a fifth Holyrood term for the Scottish National party, its leader said he would use devolved public health powers to fix prices on 20 to 50 items such as bread, milk, cheese, eggs, rice and chicken because their rising cost was “impacting our nation’s nutrition”.

The eye-catching pledge, made at the launch of the SNP manifesto for the Scottish parliament election, was immediately dismissed as a “potty gimmick” by retailers. It could also put the party on a collision course with the UK government because it may breach the Scotland Act of 1998 that created a devolved parliament.

Ewan MacDonald-Russell, deputy head of the Scottish Retail Consortium, said supermarkets already paid significantly higher rates in Scotland, £162m more over the next three years than in England.

He added: “Rather than recreating 1970s-style price controls and potty gimmicks, public policy should get serious and focus on cutting retailers’ costs so that resources can be directed to keeping prices as low as possible for customers.”

The Scottish Grocers’ Federation predicted smaller shops could be put at a competitive disadvantage, while questions were also raised about whether discounting would force supermarkets to cut the prices they pay farmers.

At the manifesto launch in Glasgow, Swinney highlighted further cost of living measures including a £2 cap on bus fares for any journey in Scotland taken on one service. The party said that would cost £210m by 2032.

The Scottish Greens, who pledged universal free bus travel in their manifesto, revealed that the SNP had rejected a similar proposal in budget negotiations last year.

Swinney claimed that an independent Scotland would be able to control energy prices, arguing that cutting them would “be on the ballot” at this election.

He was unable to name any specific policies to cut bills but said domestic electricity could be made cheaper by ending the system where the price was linked to the cost of gas used in backup power stations.

Many energy experts said that would be very difficult to achieve when gas was still an essential part of the energy market. Labour says the best way to cut home energy bills is to shift as fast as possible to renewables and nuclear power.

Speaking to supporters and candidates at an arts venue in Glasgow, Swinney said: “I want to be clear that a vote for the SNP on 7 May is a vote to put Scotland’s future in Scotland’s hands.”

He later added there was a “very realistic” possibility of a second independence referendum in 2028 but did not offer a mechanism for achieving this, given Westminster’s consistent opposition.

Swinney pitched himself as the serious candidate in a week dominated by a row between Scottish Labour and Reform over whether they could work together to keep the SNP out of power after May.

“At this moment Scotland needs experienced, principled, reliable leadership in the highest office in the land and that is what I offer this country,” Swinney said.

He pledged not to increase the number of income tax bands or rates during the next parliament. The Tories and Reform say the system is too punitive.

The manifesto, which the SNP forecast would cost an additional £1.4bn by 2032, also pledged a minimum income for artists, mirroring a scheme proposed by Scottish Labour last week, at an expected cost £30m over two years.

Other plans include the building of 110,000 affordable homes by 2032, 10% of them in rural areas and islands. This would be supported by unlocking an estimated £20bn of investable pension fund assets in Scotland.

There are also plans for a national mobile phone ban in all classrooms and for all year 1 pupils to receive a “welcome to school” bag with learning essentials such as a water bottle, books and stationery.

The 72-page document repeated earlier plans for a £100m first homes fund to give up to £10,000 in support for a deposit for first-time buyers, as well as the expansion of subsidised childcare, based on family income. Covering every child from the age of nine months to the end of primary school, it would be available 52 weeks a year.

The Institute for Fiscal Studies, an economics thinktank, said the SNP’s pledges were not backed up by a credible plan to pay for them, given that substantial public sector efficiencies had already been planned to pay for existing policies.

David Phillips, the IFS expert in devolved government spending, said this lack of precision was a “familiar pattern” in Scottish elections. “More likely in reality, paying for these plans would require further tax rises or deeper cuts to lower-priority spending,” he said.