Italian supermarket chains have forecast tens of thousands of job losses if the country’s populist government goes ahead with plans to curtail Sunday trading.

Luigi Di Maio, the deputy prime minister, said the government would introduce measures limiting Sunday trading in large shopping centres before the end of the year in an attempt to preserve family traditions.

Sunday opening hours were liberalised in 2012 by the then prime minister, Mario Monti, in an effort to prompt economic growth, despite pressure from the Catholic church, unions and small businesses to maintain the traditional day of rest.

Di Maio, also Italy’s labour minister and leader of the Five Star Movement (M5S), the party governing alongside the far-right League, said the liberalisation was “destroying Italian families”.

Politicians from both governing parties also argue that the liberal trading hours have harmed small businesses. They propose that trading should be curbed on Sundays and national holidays, with an exception in the lead-up to Christmas.

Francesco Pugliese, the chief executive of the supermarket chain Conad, told the Guardian the move could lead to between 40,000 and 50,000 job losses while bolstering online shopping, affecting both big and small retailers.

With 19.5 million Italians shopping on Sundays, the day is the second biggest of the week for revenues. Current rules allow stores to set their own Sunday opening times depending on demand.

“Blocking [Sunday shopping] at a time when the world is going in a different direction is laughable,” said Pugliese. “I always worry when the state starts telling you when to shop.”

Italy is preparing to present its 2019 financial targets, which could set it on a collision course with the EU if spending plans flout budget deficit rules.

The jobless rate fell to 10.4% in July, mostly due to more people being employed on temporary contracts, while 90,000 permanent jobs have been lost since the coalition government came to power in early June, according to figures from Istat, the national statistics agency.

Marco Marazza, a professor of labour law at Luiss University in Rome, said: “The majority of ordinary contracts include working on a Sunday, for which people are paid more, so [the measure] will impact their salaries as well as the number of those hired on purely weekend contracts.”