Striking subway workers have gone back to work in Sao Paulo, but the union involved warned they could walk out again tomorrow – the day the city hosts the opening match of Brazil's World Cup – if workers fired over the strike are not reinstated.
Union members voted last night to temporarily suspend the strike they began last week, but also decided they would take a new vote tomorrow to determine whether to resume the stoppage.
And a union representing subway workers in Rio de Janeiro said members would vote later today on whether to strike.
The actions are a severe threat for World Cup fans because the subways in both cities are being counted on as the main way for spectators to get to the stadiums.
Meanwhile, floods have driven almost 13,000 people from their homes in the southern Brazilian state of Parana, officials said.
The state's civil defence department said flooding has hit 132 cities, including the state capital of Curitiba, a World Cup host city.
It is unclear if the floods have affected Arena da Baixada stadium, which will host the first of four games on Monday. At least nine people have been killed by the floods and 30 injured, the civil defence department said.
Subway union officials in Sao Paulo met the state government yesterday afternoon but failed to reach agreement. Subway workers went on strike last Thursday and threw already congested traffic into chaos in the city of 11 million people.
Altino Prazeres, president of the union leading the strike, said almost all of the 8,000 subway employees had been off the job.
He said they were not interested in disrupting the World Cup.
"I love football. I support our national team. The point is not to stop the cup," he said. "We want to resolve this today and all are willing to negotiate."
Mr Prazeres said workers were willing to reduce their demand for a 12pc pay rise if the state-run subway company offered more benefits.
A spokeswoman for the company declined to answer questions.
Sao Paulo state officials told reporters that 42 of the striking workers had been fired.