Bank of Cyprus depositors with more than 100,000 euros (£84,300; $128,200) could lose up to 60% of their savings as part of an EU-IMF bailout restructuring move, officials say.
Up to 22.5% will go into a fund attracting no interest and may be subject to further write-offs.
The other 40% will attract interest - but this will not be paid unless the bank performs well.
It was known that the wealthiest savers at the Bank of Cyprus would take a large hit from the bailout deal - but not to this extent, the BBC's Mark Lowen reports.
Cypriot officials have also said that big depositors at Laiki - the country's second largest bank - could face an even tougher "haircut". However, no details have been released.
The officials say that Laiki will eventually be absorbed into the Bank of Cyprus.
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