Plans to send thousands of warning letters to alleged illegal downloaders have been delayed until next year as the government's embattled Digital Economy Act is held up by a high court judicial review.

Internet users across the UK were due to start receiving warning letters from their broadband provider later this year, but a legal challenge by BT and TalkTalk has significantly derailed the plans.

Under the government's controversial anti-filesharing scheme, internet service providers send notification letters to customers accused of illegal downloading by music and film companies. Repeat copyright infringers could have their internet access slowed or even blocked under secondary measures in the act.

Plans which govern the way rights owners accuse internet users were due to be published on Thursday, 31 March, but have now been delayed indefinitely by the judicial review. The already-delayed act was due to come into force in January, but now looks set to be held up until next summer at the earliest.

A spokesman for the Department for Culture, Media and Sport said that "no precise date" had been set for publication of the plans, known as the Initial Obligations Code. "We continue to work with Ofcom, and the code will be published as soon as possible," he added.

"We are working to implement the system as soon as possible. Our principal concern is to establish a system that works and provides the UK's creative industries with the tools they need to protect their hard work and investment. We will not rush this important task."

The future of the act, which was forced through parliament in the dying days of the Labour government in April last year, now rests in the hands of a high court judge.

BT and TalkTalk concluded their challenge to the proposals on Monday, arguing that the act infringes internet users' "basic rights and freedoms" and received insufficient parliamentary scrutiny.

The judge is expected to return a verdict in six to eight weeks. However, that judgment is likely to face an appeal from whichever side loses, further delaying its implementation.

The judge could also recommend that the decision be referred to the European Court of Justice, Europe's highest legal authority. In that case the act's implementation could be even further delayed.

Meanwhile, the communications minister, Ed Vaizey, is attempting to get ISPs to introduce new measures to appease the creative industries, which argue that they have been plagued by internet piracy for too long.

Among the plans, ISPs such as BT and Virgin Media are planning to launch a music-subscription service in an attempt to wean customers off illegal downloading.

Rights holders have also drawn up a list of around 100 illegal downloading sites, like the Pirate Bay and Newzbin2, that they want ISPs to block.