Unions in Brazil are threatening to strike over inadequate working conditions after two construction workers died while working at the World Cup stadium in Manaus.

Marcleudo de Melo Ferreira, 22, fell 115 feet to his death from the roof of the Arena Amazonia in the jungle city where England will play their opening match against Italy in the summer.

A few hours later, a second worker, Jose Antonio da Silva Nascimento, died of a heart attack while paving an area outside the site's convention center.

Local media claimed the 49-year-old's family had complained that he had been working seven days a week.

The state's public prosecutor's office has requested work on the Manaus stadium be suspended until safe working conditions could be guaranteed.

There remained "serious problems" at the venue regarding safety for those working high above the ground, it said.

It pleaded for all "necessary measures to prevent serious and imminent risk" to worker safety, including ensuring scaffolding was properly fixed after identifying irregularities.

"The public prosecutor's office cannot allow the urgency of finishing a construction for the 2014 World Cup to be taken care of at the cost of the life and well-being of those working on it."

Construction at the stadium has been increased with workers now operating in three shifts for all but four hours a day in a bid to ensure the stadium does not slip further behind schedule.

In all, five workers have died during the construction of the World Cup stadiums in Brazil raising fears over safety.

Three weeks ago two workers were killed when a crane fell onto the Arena Corinthians which is due to host the opening match of the tournament in Sao Paolo.