By Andrew Trotman

The German leader’s outburst came as she attempted to prove to voters she maintains a tough stance on struggling euro countries, just a month before facing key elections.

“Greece shouldn’t have been allowed into the euro,” Ms Merkel told around 1,000 supporters of her Christian Democratic Union in Rendsburg on Tuesday.

“Chancellor Schroeder accepted Greece in [in 2001] and weakened the Stability Pact, and both decisions were fundamentally wrong, and one of the starting points for our current troubles.”

Ms Merkel reiterated her desire to see a strong single currency, but warned that this can only be achieved through reforms in struggling countries such as Greece.

“That [a unified euro area] is such a treasure, such a boon, that we can’t place it in doubt,” she told her supporters. “That’s why the euro is more than a currency. For this reason we’ve shown solidarity, but solidarity always linked to responsibility for reforms in those countries that experience our solidarity.”

Ms Merkel’s finance minister, Wolfgang Schaeuble, admitted last month that Greece will need another bailout, raising fears among Germans that they will have to foot the bill.

On Sunday, the Chancellor refused to rule out another aid package but dismissed debt haircuts, which would hurt Germany as the country with the largest exposure to Greece.

“I am expressly warning against a haircut,” she said. “It could create a domino effect of uncertainty … in the eurozone.”

On Monday, Greek finance minister Yannis Stournaras said his country may have to renegotiate its bailout terms in a bid to ease its debt burden.

He told a German newspaper this could involve lower interest payments and more time to repay €240bn in loans.

Greece also faces a finance black hole of up to €10bn (£8.6bn).

The Chancellor is tipped to win a third term in the September 22 poll partly because voters approve of her management of the eurozone crisis and her tough line with struggling states.