Fears that Britain could become embroiled in Europe's deepening financial crisis intensified tonight after Spain had its debt downgraded, Portugal announced tougher austerity measures and Germany questioned whether beleaguered Greece should ever have been allowed to join monetary union.

With direct comparisons being drawn with the panic prompted by the collapse of Lehman Brothers in September 2008, European leaders promised to provide tens of billions of euros to Athens within the next three weeks. But Germany was still insisting on tougher economic reforms from Greece before approving a €100bn-€120bn (£89bn-£104bn) joint EU bailout, leading to fresh turbulence on Europe's financial markets.

Angela Merkel, Germany's chancellor, promised swift action to end the crisis, but said it was a mistake for Greece to have been allowed to join the single currency. "In 2000 we had a situation when we were confronted with the question of whether Greece should be able to join the eurozone," she said. "It turned out that the decision [in favour] may not have been scrutinised closely enough."

With the head of the Organisation for Economic Co-operation and Development, Angel Gurría, comparing the crisis to the ebola virus, the Conservatives and the Liberal Democrats warned of Greek-style problems for Britain unless swift action was taken to repair the public finances.

"This is like ebola. When you realise you have it you have to cut your leg off in order to survive," said Gurría.

George Osborne, the shadow chancellor, said: "If anyone doubts the dangers that face our country if we do not, they should look at what is happening today in Greece and in Portugal." David Miliband, the foreign secretary, accused the Tories of "economic illiteracy" over the claim, but Vince Cable, the Lib Dem Treasury spokesman, also drew the same comparison.

"The Greek position is much more serious but it is a salutary warning that unless the next government gets seriously to grips with the deficit problems, which we are determined to do, we could have a serious problem," Cable said.

At the end of a day that saw interest rates on two-year Greek bonds rise to 38% at one point, Spain joined Greece and Portugal tonight in seeing its credit rating downgraded. The announcement by rating agency Standard & Poor's that weak growth in Spain would intensify the country's budgetary difficulties reinforced fears of a cascade of sovereign debt emergencies in southern Europe with a disastrous knock-on effect on the euro.

"The crisis in Greece could pose as big a risk to the global economy and financial markets as the collapse of Lehman Brothers did in September 2008", said Julian Jessop, of Capital Economics.

He added: "It crystallises the worries about the dire state of the public finances in many countries in the same way that the collapse of Lehmans raised fears of a domino effect throughout the financial system." Lee Hardman, a currency strategist at Bank of Tokyo-Mitsubishi UFJ, said: "There is evidence of contagion from Greece. The UK's fiscal position is dire and I think the market is selling sterling on the back of that today."

Criticised for dragging their feet for months over the Greek crisis, the leaders of the 16 eurozone countries agreed to an extraordinary summit, likely on 10 or 11 May, to finalise the terms of the Greek bailout package. Dominique Strauss-Kahn, the managing director of the International Monetary Fund and Jean-Claude Trichet, the president of the European Central Bank, met Merkel and her finance minister, Wolfgang Schäuble, in Berlin yesterday in an attempt to fast-track a three-year rescue deal. Merkel, backed by German public opinion, has been hostile to footing the bill for Greece's profligacy. But amid speculation that Greece would have to restructure its €300bn debt, Schäuble said all negotiations should be wrapped up by the weekend.

He would then push enabling legislation through the German parliament next week. That would be followed the summit of the eurozone leaders. Athens would get its hands on the money before 19 May when it needs to raise €8.5bn. "The goal of the German government ... is to come to a decision as quickly as possible. The stability of the euro is the question, the last resort question," said Schäuble, a firm supporter of helping the Greeks.