“I have never seen an attack being organised so quickly,” Pascal Lamia, head of the Reporting and Analysis Centre for Information Assurance MELANI, told the Bund and Tages-Anzeiger newspapers in an interview published on Saturday.

He described as “particularly worrying” the fact that anyone can join in, even if they know nothing about information technology.

“The attack is primitive, but extremely effective.”

He said a few dozen users in Switzerland had downloaded a programme allowing them to take part in so-called distributed denial of service attacks against PostFinance - the financial services arm of Swiss Post - after it blocked an account opened by Wikileaks founder Julian Assange, on the grounds that he did not meet residence requirements.

In denial of service attacks, large numbers of computers attempt to call up a particular site simultaneously, hundreds of times a second, overloading it and totally blocking access.

Lamia called on PostFinance to make an official complaint, since that is the only way to give the authorities the right to investigate the attacks.

“People who have joined in simply out of curiosity need to know that these hacker attacks are not a game,” said Lamia.

A PostFinance spokesman told the Swiss news agency that the organisation was considering the matter. It says client data was never endangered, but admitted access had sometimes been difficult.