When the Art Gallery of Ontario in Toronto reopens its doors this autumn after a dramatic remodelling and expansion, designed by Frank Gehry and costing $200m, visitors will find an additional 2,000 works of art in the form of the Thomson Collection.

The gift of this collection, valued at about $300m and built up over half a lifetime by the media baron Kenneth Thomson – the 2nd Lord Thomson of Fleet and Northbridge (1923-2006) – represents the largest benefaction made to any Canadian institution. It also came with $50m in capital funding for the new building as well as $20m committed in endowment. First mooted in 1998 and finally announced in 2002, the gift transfers to public ownership one of the great private art collections formed in the second half of the 20th century.