The gang worked from a suburban house opposite the bank

The thieves dug a 200m (656ft) tunnel into the bank from a nearby house in the northern city of Fortaleza.

Neighbours said between six and 10 men worked at the house, rented in the name of a company making artificial turf.

The theft happened over the weekend, but was not discovered until Monday morning because the bank was closed.

Neighbours reported seeing vanloads of material being removed each day.

It took three months to dig the tunnel

The Banco Central said the robbers opened five containers with 50 real ($22) bills.

The value of the stolen bank notes has not been determined. However, police sources said the heist may have yielded as much as 150m reals, which would make it the biggest bank robbery in Brazil's history.