The Constant Gardener is a political thriller set in Africa

Ralph Fiennes and Rachel Weisz, who star in the John Le Carre thriller, were named best actor and actress.

Neil Marshall's smaller budget film The Descent took awards for best director and technical achievement.

The BIFAs honour films produced and co-produced by UK companies. Downfall, following Adolf Hitler's final days, was best foreign independent film.

The Constant Gardener is the story of a British diplomat (Fiennes) whose wife threatens to expose the pharmaceutical companies testing drugs on innocent Africans.

The awards' winners were chosen by a 13-strong jury which included actors Alan Cumming and Ashley Walters.

The most promising newcomer award went to Emily Barclay for her role in the UK and New Zealand film In My Father's Den, with Matthew MacFadyen.

Best documentary was Sean McAllister's The Liberace of Baghdad, which followed Iraqi pianist Samir Peter as he waited for his US visa while sheltering in a Baghdad hotel.

The eighth annual BIFAs were hosted in west London by actor James Nesbitt, with presenters including Hugh Dancy, Vanessa Redgrave and director Ken Loach.

Downfall follows Adolf Hitler's final days in his Berlin bunker

Rosamund Pike took the supporting actress award for her role as Elizabeth Malet in The Libertine.

Pride and Prejudice actress Keira Knightley received the Variety UK personality award, while Narnia star Tilda Swinton took the Richard Harris award, given annually to recognise outstanding achievement.

Best screenplay award was given to Frank Cottrell Boyce for Millions and the special jury prize was awarded to veteran producer Sandy Lieberson.

The Constant Gardener's success was a further triumph for UK producer Simon Channing Williams, who also worked on Vera Drake which dominated last year's BIFAs with six awards.

British Independent Film Awards 2005:

Best British independent film:
The Constant Gardener

Best actor:
Ralph Fiennes - The Constant Gardener

Best actress:
Rachel Weisz - The Constant Gardener

Best supporting actor/actress:
Rosamund Pike - The Libertine

Most promising newcomer:
Emily Barclay - In My Father's Den

Best director:
Neil Marshall - The Descent

Best screenplay:
Frank Cottrell Boyce - Millions

Douglas Hickox Award (for debut director):
Annie Griffin - Festival

Best technical achievement:
Jon Harris - The Descent

Best achievement in production:
Gypo

Best foreign film:
Downfall

Best documentary:
The Liberace of Baghdad

The Raindance Award:
Evil Aliens

Variety UK Personality of the Year:
Keira Knightley

Special Jury Prize:
Sandy Lieberson

Richard Harris Award for Outstanding Achievement:
Tilda Swinton