German chancellor Angela Merkel pledges solidarity with the Greek prime minister, George Papandreou Reuters

German negotiators are at loggerheads with their French counterparts over pledges to quadruple the eurozone's €440bn (£382bn) bailout fund ahead of a crucial vote in the Bundestag on Thursday that could decide the fate of the currency zone.

Attempts by Berlin to write off up to 50% of Greek debts as part of a wider rescue package faced stiff opposition from France, which is concerned many of its banks would need to find extra funds to cope with the resulting losses.

Wrangling over the need to expand the bailout came to the fore at a meeting of the International Monetary Fund in Washington last weekend. EU officials told the IMF plans were under way to increase the bailout fund to about €2tn and write of half of Greece's sovereign debts.

But that pledge was reported to be at risk with France said to be in conflict with Germany, Finland and Slovenia.

Fears that the major EU countries will fail to agree an expanded package of measures to cope with a default by Greece, and possibly Italy and Spain, has undermined confidence in the eurozone and led to a collapse in bank shares. French banks have suffered huge falls since the summer when concerns first emerged that they would collapse under the weight of unpaid debts if Greece defaulted.

In the last two days shares have rallied on hopes the IMF talks would lead to concrete agreement.

German chancellor Angela Merkel pledged solidarity at a meeting with her Greek counterpart, George Papandreou, but refused to reveal details of the talks and the package of measures that the markets hope will save the single currency.

The pair avoided any talk of a Greek default, orderly or otherwise, or of any multitrillion-euro rescue plan.

However, conscious of animosity among the majority of German voters about helping the less prudent nation, Merkel attached strings to her support, calling on Greece to "do its homework" in implementing painful cuts and reforms.

Several countries have demanded more stringent efforts by Greece to cut costs before the European Financial Stability Facility can be beefed up. The bailout fund's strengthened mandate must be ratified by parliaments across the eurozone.

Tomorrow the Bundestag will vote on whether to increase the powers of the EFSF – the forerunner of the permanent rescue facility, the European Stability Mechanism, due to come into force in 2013.

Three out of four Germans are against the move, which would raise the country's contributions to the pot from €123bn to €211bn. Some parliaments do not vote until the middle of next month and also face opposition from a sceptical public.

Support from Germany's opposition Social Democrat (SPD) and Green parties mean the bill is almost certain to pass. The question is whether Merkel will be able to command the so-called "chancellor majority" using only votes from her increasingly shaky coalition.

She needs 311 of her coalition's 330 MPs to vote for the bailout if she is to prove she is still in control. Failure to do so could not only trigger the collapse of the government but also harm the entire European project, analysts said. Asked by a German journalist if she was confident of getting an absolute majority in Thursday's key vote, Merkel said yes.

In an interview with Greek television channel ERT, Merkel said she was "disappointed" that the package the Bundestag was due to ratify on Thursday already appeared to be out-of-date.

"We are of course disappointed that it now looks as if the figures in September are again different from what we had expected in the programme [agreed on July 21]. Now we have to wait and see what the troika - that is, the panel of experts [from the European Central Bank, the International Monetary Fund and the European Council] - find out and what they have to say to us: do we have to renegotiate or do we not have to renegotiate? The best scenario for us is of course that the figures stay as they were [in July]. But I can't pre-empt the troika's findings."But there is scepticism. Carsten Schneider, a politician from the SPD, said it was "not acceptable" that the government was already adapting behind the scenes the very plans the Bundestag was being asked to vote on on Thursday. "Parliament will be systematically circumvented if [these] plans are not laid on the table before the vote," he said.

One rebel from the chancellor's Christian Democratic party (CDU) said he believed the bill would solve nothing.

"I'm voting 'no' on Thursday because I am of the view that in the best-case scenario, this expanded bailout fund will merely buy us time. It won't solve the problems in the long run," said Wolfgang Bosbach, an influential MP who chairs the parliament's committee on internal affairs and is not known as a eurosceptic.

"The question needs to be answered: how are we going to deal in the long term with those states in the eurozone who are hopelessly indebted and are not in the position to finance themselves?"

He expected "five to 10" other MPs from the CDU or its Bavarian sister party, the Christian Social Union (CSU), to vote against the government. Several politicians from the Free Democrats (FDP), the other coalition party, are also expected to rebel. They were meeting on Tuesday to discuss the forthcoming vote.

The Greek prime minister had travelled to Berlin to bolster support for his beleaguered nation. Papandreou gave an impassioned address to leaders of German industry, appealing to his European partners to help him tackle the country's debt crisis.

"I promise you, we Greeks will soon fight our way back to growth and prosperity after this period of pain," he said.

Papandreou said he understood the reluctance of taxpayers in other EU countries to help his country out of its crisis but said it was not an investment in the mistakes of the past, rather in the success of the future.

"The eurozone must now take bold steps toward fiscal integration to stabilise the monetary union. Let's not allow those who are betting against the euro to succeed," he said.

In her speech to the Federation of German Industries, Merkel pledged her support. "We will provide all the help desired from the German side so that Greece regains trust," she said.

"If the stability of the euro is at stake – and the experience of the last few years [tells us] that the difficulties of one country endanger our common currency – then that obliges us to show solidarity within the common currency.

"We will help if the country does all it can in terms of its own homework," she said, also reiterating her opposition to common debt issuance in the eurozone – the much-discussed eurobonds.

A poll this month showed 76% of Germans are opposed to granting any further aid to Greece and the mass-market Bild newspaper reflected public hostility to further bailouts by insisting Merkel should be tough on Papandreou.

"This is what you have to tell the Greek prime minister to his face, Frau Merkel," wrote the paper, listing demands ranging from ensuring taxes were paid to getting rid of the bloated state apparatus and "thinking the unthinkable" – namely default, a debt restructuring and even leaving the eurozone.