The government will on Friday tell internet service providers they will be hit with legal sanctions from April next year unless they take concrete steps to curb illegal downloads of music and films.

Britain would be one of the first countries in the world to impose such sanctions. Service providers say what the government wants them to do would be like asking the Royal Mail to monitor the contents of every envelope posted.

Andy Burnham, culture secretary, told the Financial Times on Thursday that the deadline was a “clear signal” of the government’s determination to tackle rampant piracy, which the music and film industries blame for the slump in CD and DVD sales.

“Let me make it absolutely clear: this is a change of tone from the government,” Mr Burnham said. “It’s definitely serious legislative intent.”

The move is a significant escalation in efforts to make internet providers take action against the estimated 6m UK broadband users who download files illegally every year.

A creative industries strategy paper published on Friday commits the government to consulting on anti-piracy legislation this spring “with a view to implementing it by April 2009”.

ISPs say it would be almost impossible to check and stop illegal downloaders. The industry has cited data-protection curbs that prevent them from inspecting the contents of data files.

The strategy paper will not set out a blueprint for how the legislation would work. “We’re saying we’ll consult on legislation, recognising there are practical questions and legitimate issues,” Mr Burnham said.

“We’re not saying ‘here’s one we made earlier, here’s a bill’.”

The concept of a “three strikes” regime of escalating sanctions, referred to in reports of a leaked early draft of the strategy, had “never been in the paper”, he said.

The Department for Business, which will lead the bill’s development, said on Thursday night that the proposal raised “difficult legal issues”, particularly in relation to European laws on online privacy and e-commerce.

But the government is adamant that the complexity of the issues involved will not prevent it from legislating if necessary. Ministers are frustrated by the slow progress of talks between ISPs and the music and film industries.

Mr Burnham said there was “no burning desire to legislate”. But he warned that ISPs could forestall statutory sanctions only if there were “considerable moves forward ... a change in the nature of the dialogue [with] good and innovative business solutions that can address the problem in a different way”.

The anti-piracy proposals are part of a series of measures set out by the government on Friday and designed to support the creative industries. These include the creation of 5,000 apprenticeships a year, involving the BBC and Tate Modern among other employers, and an annual Davos-style “world creative business conference” in the UK.