Speaking on the first day of the World Architecture Festival in Singapore, Cook told the audience he was "embarrassed" by the 67-acre redevelopment. "It's awful, boring, boring stuff," he said.
After his lecture, he told Dezeen that he found the new architecture at King's Cross "literally embarrassing" and said the buildings resembled "old biscuits".
"I can't believe it," he said. "It’s so dull, it’s like old biscuits lined up. Terrible. It’s so boring, unbelievable, really dour. It’s so worthy, like somebody wearing a sensible Harris Tweed suit but not a very stylish one. It’s awful."
Cook said the project presented a poor impression of London to visitors arriving in London on Eurostar train services, which pass through the site.
"This is London!" Cook exclaimed. "This is f*cking London. You come in from Paris and that’s the first thing you see."
The King's Cross redevelopment involves transforming former railway lands north of the King's Cross and St Pancras stations into a new urban quarter.
The project will involve the construction of 50 new buildings, 20 new streets, 10 new parks and squares and 2,00o homes by 2016, according to the kingscross.co.uk website.
Developer Argent appointed architects Allies and Morrison and Porphyrios Associates to draw up a masterplan for the site, which won planning permission in 2006.
Buildings completed on the site so far include the new campus for Central St Martins art and design school designed by Stanton Williams, apartment blocks by David Chipperfield and Maccreanor Lavington, an office building by Allies and Morrison, student housing by Glenn Howells, social housing by PRP and a freshwater bathing pond.
Thomas Heatherwick also recently revealed an image of his design for a shopping centre to be built in a former Victorian coal yard, and is also working on plans for a new headquarters for Google nearby. King's Cross rail and underground station was recently revamped by John McAslan, and St Pancras.
Cook was speaking on the first day of this year's WAF, which is taking place this week at the Marina Bay Sands convention centre in Singapore.
In his lecture, Cook praised recent architecture in Singapore and described landmark buildings including Moshe Safdie's Marina Bay Sands hotel and Wilkinson Eyre's conservatories at Gardens by the Bay as "heroic".
"Audiences such as us have got bored with the word 'iconic'," he said. "So I’m using 'heroic'."
Cook said that King's Cross didn't need to feature similar "high-jinks" architecture but said: "It seems to be set on being grey and grim. It could be elegant architecture."
Cook, 79, was a member of influential 1960s architecture group Archigram and is now co-director of Crab Studio, which he founded with Gavin Robotham in 2006. He won the RIBA Gold Medal in 2004 and was knighted for services to architecture in 2007.
Crab Studio projects include the Abedian School of Architecture at Bond University in Queensland, Australia. The building won the Health and Education category at last year's Inside Festival, which was held in Singapore alongside WAF 2014.