Greek bank shares suffered their worst one-day loss on record yesterday as anxiety grew over the new government’s plan to renegotiate the country’s €240 billion bailout.

The country's four biggest lenders saw their stock prices plummet by an average of more than 25 per cent just two days after Alexis Tsipras, leader of left-wing party Syriza, was sworn in as prime minister.

It was the third day of double-digit share slides for the banks.

A sell-off in Greek debt also accelerated yesterday, sending short-term borrowing costs to their highest level since Greece’s 2012 debt restructuring.

In the space of a few hours, the yield on three-year Greek bonds jumped 2 percentage points to almost 17 per cent, as investors wondered whether Greece would honour its debts in the near term.

Financial markets have been febrile ever since Syriza, which is seeking a write-off of some of Greece’s debts and an end to the austerity policies imposed on the country by its international creditors, won Sunday’s parliamentary elections.

Alberto Gallo, head of European credit strategy at RBS, said investors feared the relationship between Greece’s new government and its creditors would be “very confrontational”.

“If the government threatens debt restructuring it scares creditors but it also affects their own system, especially banks. It’s like playing with nuclear weapons – if used, then the threat would destroy both parties,” he said.

Mr Tsipras told his first cabinet meeting that Greece would not bow to international pressure. In a message to supporters who backed his party’s anti-austerity platform, the prime minister said ministers “must not disappoint the voters who gave us a mandate”.

“We are coming in to change radically the way that policies and administration are conducted in this country,” he said. – Copyright The Financial Times Limited 2015