A bailout of struggling Italian bank Monte dei Paschi should not be seen as a done deal, a key European Central Bank policymaker has said.

Jens Weidmann, president of Germany’s Bundesbank, said the Italian government ought to consider whether it should rescue the bank if it is in a bad financial state.

“For the measures planned by the Italian government the bank has to be financially healthy at its core,” he said in an article published German newspaper Bild.

“The money cannot be used to cover losses that are already expected. All this must be carefully examined,” added Weidmann, a member of the ECB’s governing council.

The ECB has taken a tough line with Italy’s third-largest bank, refusing to give it more time to find private investment. The Italian government approved a state bailout plan last Friday after the Siena-based bank, which can trace its roots back to 1472, failed to convince investors to fund a €5bn (£4.25bn) cash injection.

It plans to dip into a €20bn fund approved by the government earlier this month for the purpose of propping the bank up if no rescue plan could be secured elsewhere. The bailout plan was spurred by Monte dei Paschi’s poor results in ECB “stress tests”, a system set up by the European Union after the financial crisis to measure banks’ resilience to unexpected economic shocks.

Stress tests are intended to ensure that taxpayers are not asked to pick up the tab for multi-billion pound bailouts, such as those that the British government orchestrated to save Lloyds and Royal Bank of Scotland. Fears about the strength of Monte dei Paschi began to mount after it came last out of 51 European banks in the ECB’s latest round of tests in July. The bank was told to shore up its finances by the end of the year or face being wound down.

“These (rules) are meant especially to protect taxpayers and put responsibility on investors. State funds are only intended as a last resort, and that is why the bar is set high,” Weidmann told Bild.

Monte dei Paschi’s problems have piled up since the 2008 banking crisis, when it paid €9bn (£7.6bn) for Banca Antonveneta.

The deal doubled its size and turned it into Italy’s third largest bank behind UniCredit and Intesa Sanpaolo. It bought Banca Atonveneta from Santander, which had acquired the Italian bank during the three-way bid for Dutch bank ABN Amro, a deal that was a key factor in the taxpayer bailout of Royal Bank of Scotland.

Three years ago Monte dei Paschi’s problems escalated. The Sienese bank asked the government for €4bn amid a scandal over loss-making derivatives contracts and alleged fraud. It has gone back to the the government for funds after talks with private investors failed, with Qatar’s sovereign wealth fund thought to have scuppered the plan by refusing to take part. The Italian government already has a 4% stake in Monte dei Paschi.