In the ten years that they have been away, it is easy to believe that something terrible has happened to the three core members of Portishead. Where once their music was suffused with a sense of melancholy, of impending doom, Third suggests that doom has arrived. If this extraordinary album had been made by a new band, major labels would not let these agents of horror into the building. Radiohead were probably the last hugely popular band to charge so vigorously towards the avant-garde. But even Kid A feels approachable compared with Portishead's uncompromising third album.

Portishead, of course, were never exactly an easy-listening band. In Beth Gibbons, they had an exquisitely neurotic soul singer, one whose vocal agonies could make Amy Winehouse at least sound well adjusted. In Sour Times, their biggest hit, they had a song that compelled their millions of fans to cheerfully join in a chorus of “Nobody loves me, it's true”.

But thanks to their love of old soundtracks, their cunning deployment of vinyl crackle, there was also something warm and nostalgic about the Portishead sound. Third gleefully dispenses with such niceties. Its arrival is heralded by Machine Gun, a single of thrilling brutality, built on jarring synths and sputtering industrial beats. In the midst of grinding dystopia, Gibbons sounds utterly forlorn. There is a “poison in my heart”, we learn, unsurprisingly.

Occasionally, a song such as Plastic will conjure up the ghosts of old Portishead, only to assail them with great looming slabs of noise. But prettiness does exist here. The Rip begins as a dappled pastorale, before keyboards overwhelm the acoustic guitar, and the song turns into the sort of humming marvel first synthesized by Kraftwerk.

Nevertheless, the prevailing mood of Third is typified by its ending: a reverberant, apocalyptic guitar chord that betrays Portishead's love of leftfield heavy metal bands such as Sunn O))). It is these dark adventurers whom they desire to be classified with now, rather than trip-hop lightweights such as Morcheeba. And it is these bands who have, implausibly, inspired Portishead to make the bravest, weirdest and best album of their career.

(Island, TMS £11.99, call 0845 6026328)

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