Shares in troubled Portuguese bank Banco Espirito Santo have plunged by more than 50 percent in volatile early trading on the Lisbon stock exchange a day after it reported a record half-year loss of 3.58 billion euros ($4.8 billion).

The bank's share price, which four months ago was around 1.3 euros, fell as low as 0.17 euros Thursday.

The fall began after an audit discovered accounting irregularities at the bank's parent company and gathered momentum after three of the Espirito Santo family's holding companies requested bankruptcy protection. Police suspect the former chief executive of fraud and forgery.

The bank's new board said its recovery plan will include a recapitalization and asset sale.

The Bank of Portugal stripped the Espirito Santo family's holding company of its voting rights at the bank.