Brazil's Congress has approved a constitutional amendment that will cut the working week from 44 hours to 40 and give employees two days off per week without cutting their salary.

The change represents the biggest overhaul to workers' rights since Brazil's return to democracy in 1988.

Senators passed the amendment, which left-wing President Luiz Inácio Lula da Silva had championed, less than two weeks before a second round in the Latin American nation's presidential election.

Lula's supporters are hoping that its passage, which is estimated to benefit more than 30 million workers, will boost his chances of closing in on his right-wing rival, Flávio Bolsonaro, in the 25 October run-off.

The son of former President Jair Bolsonaro won the first round of the presidential election with 47% of the vote, ahead of Lula on 45.2%. Both fell short of the 50% of valid votes needed to stave off a decisive second round.

Opinion polls suggest that the move to cut the working week is very popular, with one nationwide survey finding that almost 70% of Brazilians backed it.

The measure had already passed in the Chamber of Deputies, Brazil's lower house, in May, before senators overwhelmingly backed the measure on Wednesday by 75 votes to one.

Flávio Bolsonaro, who is currently a senator, dropped his previous opposition to the measure and also cast a "yes" vote.

Announcing his change of heart on Tuesday, he argued that he had been swayed by the measure's popularity, commenting: "Most Brazilian workers ‌want to ⁠see the end of the six-day workweek, so I'm going to vote in favour of ending it."

The younger Bolsonaro had earlier denounced the proposal as an "opportunistic" attempt by his presidential rival to appeal to working-class voters.

Before his about-turn, he had said that it should be down to employees and employers to agree on a work schedule, rather than restrictions being imposed by a constitutional amendment.

Lula described Bolsonaro's policy reversal as "electoral opportunism" and celebrated the amendment's approval, saying it would make a real difference to workers.

"It means more time to live, rest, be with family, watch your children grow up, look after yourself and enjoy life," he wrote on social media.

Critics of the measure have warned that it will raise production costs, which will eventually be passed on to the consumer, resulting in higher prices.

But its supporters argue that having two days off a week will allow workers to be better rested and therefore more productive in the hours that they do work.

The change is due to be phased in, with the working week first being shortened to 42 hours after a two-month grace period and then again to 40 hours at a later stage.

The changes will take more than a year to entirely take effect but many of its backers are already celebrating.

Luciana de Almeida, 36, who works six days a week as a cashier in a supermarket, said it would "make a lot of difference" to her life.

The single mother, who lives in Rio de Janeiro, said she has been missing out on time with her three children as the only day off she had was usually spent doing domestic chores.

"My body, and my mind in particular, are very tired," she told BBC Brasil.

"I need time with my daughters, have some leisure time, or even just some time resting at home."