MoneySupermarket says that 18% of internet users had previously illegally downloaded music, films, games, software or television programmes.

While more than a third believe access of high-speed broadband services will encourage greater piracy and make it easier to illegally download.

"Illegal downloading is already a big problem for the likes of the music and film industries," said Mike Wilson, broadband manager at MoneySupermarket. "With superfast broadband packages set to become commonplace, the problem seems likely to get worse."

The company also warns that the new Digital Economy Act might lead to home owners being falsely accused of piracy. Wi-Fi hijacking, in which other people use unsecured wireless networks to access the internet and illegally download content, could be one of the biggest causes of mistaken identity. The survey found that 19% of wireless users have not secured their home internet connection with a password.

"Those who suffer Wi-Fi hijacking or share a connection with multiple users should not be penalised for the actions of others," said Wilson. "But it is imperative that people take appropriate measures to protect their wireless connections."

The police have warned home owners that some fraudsters are using the Digital Economy Act to con people out of money. Under the terms of the Act, letters will be sent to people suspected of illegally downloading material online, threatening them with possible disconnection.

Fraudsters are sending letters to households claiming that their internet connection has been used for illegal downloading, and that they face huge fines and possible prosecution if they do not pay a £35 fine within seven days. The official-looking letter is sent by a fake company called HSE Business Support, and is headed "Unlawful Activities".

"Anyone who receives a letter matching this description is advised not to pay any money and report the matter," said Duncan Smith, a spokesman for Hampshire and Isle of Wight Constabulary.