Campaigners including the writers Philip Pullman and Zadie Smith have lost their appeal to save six libraries from being closed by Brent council in London.

In what has been regarded as a test case, three judges at the court of appeal rejected claims by campaigners that the closure was "fundamentally flawed and unlawful", leading to fears that the "treasured" buildings could now be permanently closed.

The campaign group Brent SOS Libraries will now seek permission to appeal to the supreme court. "This is such an important issue – we don't believe the judge addressed the very real concerns that will be relevant to local authorities and library users and vulnerable groups up and down the country, so we will carry on," said the group's spokesperson Samantha Warrington.

She added that she hoped the court would agree to an appeal before Brent council moved to dispose of the libraries permanently. The group has already mounted vigils outside two of the libraries, Preston Road and Kensal Rise, to ensure that books and computers were not removed by the council.

The battle over Brent's libraries is viewed as a landmark case as councils implement cuts. In October, Pullman said the council's statement claiming that the closures would help "improve" its library services "ought to be quoted in every anthology of political bullshit from here to eternity."