Mihail Mikov, who took over as party chief in July, told Reuters no taxpayer money could be spent on rescuing Corporate Commercial Bank (Corpbank) until the full extent of its financial problems were revealed in an audit, due by Oct. 20.

Corpbank's closure has left hundred of thousands of depositors with no access to their accounts. The bank also failed to make the final payment on a $150 million bond that matured in August, prompting threats of legal action and raising a new deterrent to foreigners considering investment in the European Union's poorest state.

With a general election set for Oct. 5, the Corpbank crisis has fed Bulgarians' already high levels of disillusionment with their political leaders, which could lead to a fractured vote and another fragile government struggling to implement reforms.

Mikov joined calls for Bulgaria's central bank governor to resign over Corpbank's collapse but said nothing could be done yet for investors or creditors. "We have no right to burden taxpayers without knowing what we are faced with," he said in an interview.

Corpbank, Bulgaria's fourth largest lender, has been in limbo since June 20, when a run on deposits prompted the central bank to seize control and close its operations, triggering the worst banking crisis in the Black Sea state since the 1990s.

Political bickering has bogged down a central bank plan for a state rescue. The central bank has extended its controls over Corpbank until Nov. 20 saying the next parliament should decide on its fate, pending the results of a full audit and outcome of talks with current shareholders.

"It should have been clear by now what are the bank's obligations, its dues, its collateral... After all that has happened, it is obvious that the trust in the banking system cannot be restored with this governor," Mikov said.

Other political leaders have also urged central bank governor Ivan Iskrov to step down over Corpbank.

Iskrov has said technical problems have delayed the results of the audit. He has declined to say whether he would resign, but has indicated that attempts to oust him from the post may breach EU rules protecting central bank independence.

The Socialists, whose government resigned in July after one year in office, are trailing the centre-right GERB party in opinion polls. Their term was marked by sustained anti-graft protests, devastating floods and the banking crisis.

"Bulgarians need political calm and more justice.... It is necessary to have a bigger (parliamentary) majority, but only based on clear principles. Otherwise it will not get public support," Mikov said.

The Socialists will seek legal changes to allow tougher controls over banks and change the flat income tax rate, currently at 10 percent with a graded system.

(Reporting by Tsvetelia Tsolova; Editing by Ruth Pitchford)

By Tsvetelia Tsolova