Some of Greece's biggest banks may have to shut and merge with their rivals, it has been claimed.
A European official told the Reuters news agency that Greece's four big banks - National Bank of Greece, Eurobank, Piraeus and Alpha Bank - could be reduced to just two.
European leaders are meeting on Sunday in a bid to strike a financial aid deal with Greece.
David Cameron is to chair a meeting this morning in which he will review contingency plans already in place over Greece's decision to reject terms of an international bailout plan.
A Number 10 spokesman said: "The Greek people expressed a decisive view in yesterday's referendum. This is a critical moment in the economic crisis in Greece. We will continue to do whatever is necessary to protect our economic security at this uncertain time.
"We have already got contingency plans in place and later this morning the Prime Minister will chair a further meeting to review those plans in light of yesterday's result."
The Government has also issued travel advice for British tourists, warning banking services, such as credit card purchase and cash withdrawals, could become limited at short notice.
The Greeks have voted to reject the austerity terms demanded by eurozone creditors - so, what happens next?
Greeks have voted to reject the terms of an international bailout, putting it on the brink of a eurozone exit.
Greece's controversial Finance Minister Yanis Varoufakis has released a defiant statement of resignation entitled 'Minister no more!'