A co-leader of the rightwing populist Alternative für Deutschland party is facing calls for her resignation over donations received by her campaign before Germany’s general election last autumn.

Alice Weidel’s office near Lake Constance, in southern Germany, received a total of €130,000 (£110,000) in individual payments of 9,000 Swiss francs between July and September 2017.

A joint investigation by the German broadcasters WDR and NDR and the newspaper Süddeutsche Zeitung traced the donations to a Swiss pharmaceutical company. The payment reference was reportedly “election campaign donation for Alice Weidel”.

Weidel is the most public face of the anti-immigration party and has repeatedly highlighted the AfD’s aim to raise moral standards in German politics.

She said she had paid back the money immediately on learning of the payments. She denies wrongdoing and has ruled out resigning.

But the investigation found that the money was not repaid until April this year and was around €6,000 short of the original amount received.

Under German law, parties can receive donations from non-EU countries only if they are made by German citizens. A party is obliged to inform the lower house of parliament of any single donations above €50,000.

If the rule is contravened the financial penalty is set at three times the amount of the donation – which in this case would be €390,000, said the deputy leader of the Bundestag, Wolfgang Kubicki. He said he struggled to understand how the party had not realised such donations were illegal.

“Those in a position of responsibility in the party must have known that donations from non-EU countries are to be paid back immediately or are to be handed over to the leader of the Bundestag,” he said.

AfD’s other co-leader, Alexander Gauland, admitted that the money had been paid back too late but came to Weidel’s defence. “I don’t believe that she should be blamed for anything,” he told the tabloid Bild, accusing a party treasurer of wrongdoing instead.

The German branch of the anti-corruption NGO Transparency International said it was doubtful about Weidel’s version of events. “Frau Weidel will have to explain and lay bare when she and her district office found out about the inadmissible major contribution and which measures they took to deal with it,” Hartmut Bäumer, the deputy head of Transparency Deutschland, told the Passauer Neue Presse.

The AfD entered the Bundestag for the first time last October and is currently polling at 17%, second to Angela Merkel’s CDU. It has grown in popularity firstly as an anti-European party when it was founded in 2013 and latterly as an anti-Islam, anti-Merkel force.