A senior court judge has pointed to severe problems with the way the Digital Economy Act enables copyright owners to accuse people of illegal filesharing.

Judge Birss QC said on Tuesday that the process of connecting copyright infringement to a named individual based on their use of an internet address is fraught with difficulties because internet connections, or IP addresses, are often used by more than one person.

In a ruling in the patents county court, Birss asked: "Does the process of identifying an IP address in this way establish that any infringement of copyright has taken place by anyone related to that IP address at all?"

He said that the assertion did not hold up: "Even if it is proof of infringement by somebody, merely identifying that an IP address has been involved with infringement [does not make it] clear to me that the person identified must be infringing one way or another. The fact that someone may have infringed does not mean the particular named defendant has done so."

The use of "unsecured" internet connections which allow others to "piggyback" on their network leads to more complications, Birss said, adding that these issues are "key" in proving copyright infringement before a court of law.

That could create serious problems for copyright owners seeking to enforce their rights under the Digital Economy Act. Although the law allows for a "three strikes" provision in which internet service providers (ISPs) would be required to write to the people who are using an IP address at a time that it is found to be infringing, it has not yet been implemented. Beyond that, copyright owners might be able to take people to court or demand that their internet connection is throttled or constrained.

But Birss pointed to a lack of case law to support the idea that someone in charge of an IP address could be judged guilty of infringement carried out through that address.

"What if the defendant authorises another to use their internet connection in general and, unknown to them, the authorised user uses P2P [peer-to-peer] software and infringes copyright?" he noted in his ruling.

"Does the act of authorising use of an internet connection turn the person doing the authorising into a person authorising the infringement within s16(2) [of the Copyright, Designs and Patents Act]? I am not aware of a case which decides that question either."

The BPI, which represents record labels in the UK, has successfully prosecuted a number of people in the UK after showing that they were using P2P software and downloaded files from them which it could present as evidence. However no such evidence was presented in the cases being considered by Birss, and it is not clear that the Digital Economy Act requires that level of evidence either for ISPs, which are asked to write letters to alleged infringers.

The ruling relates to 27 cases of alleged illicit filesharing brought by ACS:Law and the copyright licensee MediaCAT, could have implications for the Digital Economy Act. Under the act, the UK's largest internet service providers will send warning letters to internet users whose IP addresses are linked to illegal downloading, and anyone who receives three letters in 12 months could be sued by the copyright owner.

ACS:Law, the controversial law firm at the centre of a huge row over illegal downloading and data protection, on Tuesday had its attempt to discontinue lawsuits against 27 people it suspected of illegal downloading quashed by the court.

Birss said that the letters demanding payment from alleged infringers "materially overstates the untested merits" of the companies' approach to proving illicit filesharing. These cases are the first to be taken to court from the tens of thousands of letters sent out by ACS:Law on behalf of MediaCAT.

ACS:Law and MediaCAT shut down their respective operations last week, just days before today's key court decision.

The two companies have 14 days to involve the copyright owners in the 27 proceedings, otherwise the cases will be dropped and the claimants liable for outstanding legal costs.

Michael Forrester, a solicitor at the Manchester-based law firm Ralli which is representing five of the 27 defendants, said: "The judgment highlights a number of legal and technical difficulties with these cases which we had advised our clients of throughout.

"We are dealing with cases where consumers have explained how they cannot possibly have uploaded or downloaded copyright protected material, but they are still being pursued."

A DCMS spokesman said: "The measures in the Digital Economy Act do not aim to attribute liability for online copyright infringement. They aim to reduce and prevent such infringement.

"Under the mass notification system internet account holders will be informed when infringement is happening on their account and given advice on how to prevent it. The letters will aim to inform and will not ask for money.

"The code, which is due to be published by Ofcom shortly, will require a robust standard of evidence and there will be a readily accessible appeals process.

"The DEA and the code will not affect the standards by which courts make judgements about copyright infringement when rights owners seek to enforce their rights through the courts."

• This story was amended on 10 February 2011 to clarify the first line and add a statement from the DCMS