Studio chiefs at Warner Brothers believe costs for the lavish production – which will be shot entirely in Britain – are likely to soar as high as a record-breaking £75million per episode. With plans for seven seasons – one for each of JK Rowling’s beloved books – of six episodes each, that would send the overall total to a mind-boggling £3.15billion, making it the most expensive show in television history.

Producers have revealed stars John Lithgow as Albus Dumbledore, Paapa Essiedu as Severus Snape, Janet McTeer as Minerva McGonagall, and Nick Frost as Rubeus Hagrid. Luke Thallon will portray Quirinus Quirrell, and Paul Whitehouse will be Argus Filch. They have also watched 32,000 audition tapes after an open casting call for the roles of Harry, Ron and Hermione, played by Daniel Radcliffe, Rupert Grint and Emma Watson in the film series.

They are expected to announce their choices next month. A senior Hollywood production source confirmed: “There’s no question Harry Potter will become the biggest TV show the world has ever seen – as well as providing an enormous windfall for the UK.” Shooting is due to start this summer at the 200-acre Warner Bros studio complex at Leavesden, Hertfordshire.

The TV adaptation is expected to launch next year across HBO cable and streaming platforms. The source added: “It’s going to generate well-paid jobs for several years as well as ensuring Britain remains one of the world’s most important filming hubs.”

As the Sunday Express revealed in February, so many filmmakers and TV productions are heading to the UK – due to tax incentives and spiralling costs in the US – that it is now dubbed “Hollywood East” in industry circles. Harry Potter on TV, however, will easily be the biggest ever financial juggernaut, with executives believing profits may eclipse the £7.4billion made by the eight films and the Fantastic Beasts spin-offs.