The sources said planning had been going on for several weeks and was designed to ensure Greece gets the liquidity needed to avoid default in the absence of the next, €12-billion tranche of its emergency loan package, due by mid-July.
As well as preventing default, the aim is to head off any contagion spreading from Greece to Ireland, Portugal and Spain, and the potential knock-on impact on Europe's banking system, with French and German banks large holders of Greek debt.
The plan is distinct from a French proposal for private sector involvement in a second Greek bailout programme and is being discussed despite European Commission President José Manuel Barroso and other senior EU officials repeatedly saying that "there is no Plan B for Greece".
"There's been thinking about contingency for some time, for several weeks," one senior eurozone finance official involved in the Greek bailout told Reuters. The other sources seconded that line, saying there was "active planning" to step in if the Greek parliament rejects the austerity programme (see 'Background').
"In this sort of situation, you can't afford not to think about what might happen next," the first source said.
The sources would not confirm in detail what the current plan involved, but said several options had already been dismissed, including an EU bridging loan to Athens.
"This option was discussed a few days ago, before the Eurogroup meeting in Luxembourg [on 19-20 June], but I understand it's now been ruled out," a second source said.
They would not be drawn on what action banks might take, but British Prime Minister David Cameron made clear at an EU summit last week that banks needed to strengthen their balance sheets and be ready for any potential fallout from Greece.
"All European countries need to use the time that we have to strengthen banks and bank balance sheets and make sure they are meeting all of the requirements so that they are strong and can withstand any problems," he said.
"Banks right across Europe that have exposure to Greece [...] every bank needs to make absolutely clear what its exposure is."
Repayments due
Greece has to roll over €2.4 billion of six-month treasury bills on 15 July and €2.0 billion of three-month bills a week later. In August, it has €5.9 billion of five-year bonds maturing and must roll over €2.5 billion of bills.
Failure to refinance the debts would result in the first default since the euro zone was created in 1999.
One of the sources said the most likely way a default would be avoided in that case would be the extension of another loan from a third party, although they would not go into details.
"I find it difficult to believe that no bank, no institution, would be ready to extend Greece a further loan," the source said. "At the right terms, it will be done."
China has been buying eurozone debt and Chinese officials have said they intend to continue that trend. Chinese sources say the country remains interested in extending its commercial investment in Europe, such as taking stakes in companies.
'Necessary evil'
Greek Prime Minister George Papandreou on Monday called on all political parties to back an unpopular five-year austerity plan, which international lenders have made a condition for further aid to Greece.
"Our vote is the only chance for the country to get back on its feet," Papandreou told legislators at the beginning of a parliamentary debate on the law.
He said a vote in the favour of the law would end the uncertainty hanging over Greece.
Efstathios Koutmeridis of the governing Socialists called the vote "a necessary evil".
Reportedly, one Socialist politician has threatened to vote against the measures, but one conservative opposition MP has pledged to support the government.
At a recent European People's Party summit, leaders from centre-right European parties failed to convince Antonis Samaras, the leader of the opposition New Democracy party, that he should support the austerity programme.
EurActiv with Reuters
