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The purists could be up in arms, but perhaps there won't be too many of them. Taking its cue from the Metropolitan Opera in New York, the will on Thursday embark on an ambitious experiment. It will launch its pilot season of NT Live, broadcasting live performances of plays onto cinema screens in the UK and around the world.

Nearly 300 cinemas around the world (73 in the UK) will see a live performance of (in a translation by Ted Hughes), with Helen Mirren in the title role. This performance will be beamed live via satellite from the National theatre, using five cameras.

Phedre is the story of a woman consumed by an uncontrollable passion for her young stepson - and thinking her absent husband, Theseus, to be dead, confesses her darkest desires.

When I asked the director Nick Hytner why he chose this play, he said two words: "Helen Mirren".

If this could work with anyone, it would work with her selling it - and she does, in trailers in cinemas up and down the country, mainstream, selective Odeons and Vue cinemas, as well as more art house venues. And why this play? It is, according to Hytner, a purely theatrical experience, but will not and should not be viewed as a film for those sitting in cinemas.

It is more like the equivalent of watching a live sporting event. Phedre is certainly an intense evening at the theatre, and there isn't much in the way of action; it is emotional and requires a serious attentiveness.

There has always been an issue with filming theatrical productions; directors tend to hate it because it makes the productions look flat. But Hytner is a convert in this instance, for two reasons: he thinks that the National can - and should - reach as wide an audience as possible; and the technology, he believes, gives a clarity and resolution which makes it feel "live" for the cinema-going audience.

Subsequent productions coming to a cinema screen near you will be Shakespeare's All's Well that Ends Well; Nation, based on a Terry Prachett novel and adapted by Mark Ravenhill, and Alan Bennett's new play, The Habit of Art, with Michael Gambon, Alex Jennings and Frances de la Tour.

The Met's experiment is hugely successful. Opera is beamed live into 800-plus cinemas around the world. But it costs money. Each production the National does this with will cost £120,000 and, although it is sponsored, the National hopes tickets sales will be able to sustain it in the longer term.

The reach of the National Theatre's productions will certainly increase; ticket sales for the first Phedre cinema performance are currently 71%. But will there be a cost in terms of quality of experience? And is that just a precious perspective? Of course, making artistic work as widely accessible as possible, especially when funded by the taxpayer, is a good thing. But is it being done at the expense of the actual experience of the art form? And would it be complacent to accept that theatre will always be for a niche audience?

In some respects this is a good example of how innovative the creative industries can be; using technology to stay relevant and create new audiences, and the proof, as ever will be with the number of bums on seats.