Chief Executive Vitor Bento, finance director Joao Moreira Rato and deputy CEO Jose Honorio said they were leaving because their initial mandate to revive the bank with private money had changed too much since the government bailout.

BES, Portugal's largest listed lender, had to be rescued after the collapse of the business empire of its founding Espirito Santo family, whose main holding firms are under creditor protection. Analysts and auditors have warned some losses and liabilities may yet be uncovered at the bank.

The management resignations followed a reported disagreement with the central bank over Novo Banco's long-term strategy.

Weekly newspaper Expresso said earlier on Saturday, before the resignations were announced, that the executives had objected to the central bank's plan to sell Novo Banco as soon as possible to recover the 3.9 billion euros ($5.1 billion) in public funds used in the rescue.

The men were reluctant "to run a project that is not theirs", said the paper, which did not name its sources.

However, the three men denied any conflict.

"Our decision to resign is due to the fact that our mandate significantly changed since we began our roles, in mid-July. During our time at Novo Banco, we have contributed to the stabilization of the bank, taken steps to normalize operations and improve systems and launched a medium-term plan," they said in a joint statement.

Bento's initial mandate was to save BES using private capital, before the central bank decided on Aug. 3 to step in with 4.9 billion euros, some of which was provided by Portugal's bank resolution fund, to recapitalize the "good bank" Novo Banco that it carved out of the troubled BES.

"A process for a swift sale of the bank has already been launched and is being managed by the Resolution Fund and the Bank of Portugal. We now feel that the right thing to do is to hand over the reins to another management team," Bento said.

SPLIT

Expresso said the executives had offered to resign previously as they were against the central bank-ordered split of BES into Novo Banco and the bad bank that inherited the exposure to the Espirito Santos' troubled web of businesses.

Bento said he and his team believed Novo Banco was "a great institution, with dedicated people, loyal customers and a business that can strongly contribute to lead Portugal through the recovery".

He said his early announcement of his intention to resign gave time to prepare a smooth transition to new management.

The Bank of Portugal said it would soon name new management for Novo Banco, and reiterated its intention to sell the bank to investors "within the shortest reasonably demandable period" to guarantee a stable ownership structure.

In a statement, it thanked the executives for having stayed on at Novo Banco after their initial mandate was "invalidated by the resolution of BES". Local media said the regulator was likely to announce Bento's replacement this weekend.

Bento, a respected economist and business manager, was picked by Bank of Portugal Governor Carlos Costa to replace Ricardo Salgado, the patriarch of the Espirito Santo clan that founded the bank some 150 years ago. Salgado had resigned under pressure from Costa.

Bento's new management and the Bank of Portugal have said they suspect illegal activity took place at BES, involving the financing of the troubled family businesses using money borrowed from BES clients.

BES and Novo Banco have already encountered difficulties as they try to recover some of the Espirito Santo family's massive debts, with some investors and a family holding company trying to block the sale of insurer Tranquilidade.

Earlier this month, auditor KPMG warned of possible further losses on top of a huge 3.6 billion euro loss reported by BES in July.

(1 US dollar = 0.7716 euro)

(Editing by Mark Potter and Pravin Char)

By Andrei Khalip