The $50.8m allowance comes on top of the UK Film Council's annual funding.
Purnell announced the funding package today ahead of his appearance tonight at the LFF's opening gala and screening of David Cronenberg's Eastern Promises. The British Film Institute archives have been the subject of increasing controversy in the UK in recent months due to industry and public outcry that the nation's film heritage was in jeopardy.
The new funding is in addition to $6.1m (£3m) from the UK Film Council for the UK Digital Film Archive Fund.
The UK Film Council will use the funds to presever and restore the BFI national collection and regional collections; put in place a strategic approach to safekeeping the collections and navigating rights and digitisation; and increasing public access to the archives, including regional access.
The BFI National Collection holds more than 60,000 fiction features, 120,000 non-fiction titles and 675,000 TV programmes. In total, it represents more than 500,000 hours of material.
But as the content deteriorates, more funding has been needed for restoration. An estimated 30% of the BFI's acetate collection is deteriorating.
'The archive is a national treasure. It's a visual history of Britain since the moving image began. It's vital that we safeguard its future,' Purnell said. 'This additional £25 million will secure the future of the national and regional archives. It's absolutely right that they should be safe and accessible for future generations.'
John Woodward, CEO of the UK Film Council, said: 'We are now in a position to take forward our plan for screen heritage in the UK which has been developed in partnership with the sector.'
Amanda Nevill, director of the BFI, added: 'Through our emerging and nascent projects such as TV co-productions, online access activities and the Mediatheque, we have proved just how hungry the public is for archive and heritage film and how much they value it. This level of investment will mean we can once again set a world standard in conservation and preservation and bring into view so much more of our precious heritage captured on film and that the public is clamouring for.'
In addition to UKFC funding, the BFI also draws from funding partners such as HP and the Heritage Lottery Fund.