Eurozone leaders most show the world they are prepared to stand behind their currency, Chancellor George Osborne has said.
Arriving in Brussels for a meeting of EU finance ministers, he said that they needed to build a convincing "firewall" to prevent the crisis spreading.
He warned that failure to build on last month's deal to bail out the stricken Greek economy would have a "very damaging effect" on Britain.
"We need to focus on getting a firewall in place. It is all very well saying we have got a firewall, but the eurozone now need to convincingly show the world that the firewall exists and has got sufficient resources in it," he told reporters.
"The eurozone needs to show the world that it can stand behind its currency. We can't just stand and wait on developments in Athens and in Rome. We have also got to make progress here in Brussels.
"If we don't, that will continue to have a very damaging effect on the entire European economy, including the British economy."
The plan's supporters resisted Mr Osborne's challenge to hold an immediate vote, and the proposal was still on the table when the Chancellor returned to London.
The Commission stood firm on the need for the tax.
"We just need to look at the demonstrations - across the EU and across the world - to see how greatly citizens want the financial sector to make a proper contribution to the economy and to society as a whole," said EU taxation Commissioner Algirdas Semeta.
Max Mawson, spokesman for the Robin Hood Tax campaign, said: "George Osborne appears determined to ignore the positives and exaggerate potential negative impacts of financial transaction taxes."