Police are investigating the possibility that arsonists set fire to the factory opposite White Hart Lane that Tottenham Hotspur were planning to move to.
London Fire Brigade was called to the Archway Sheet Metal Works, on Paxton Lane, shortly after 5.15am on Tuesday.
Six fire engines and 35 firefighters tackled the blaze, which had consumed half of the ground floor and the whole of the first floor.
The Metropolitan Police are now appealing for witnesses after describing the fire as "suspicious".
"At this early stage the fire is being treated as suspicious," a Met statement said.
The owners of the factory previously reported receiving death threats, supposedly from Tottenham Hotspur fans, over their opposition to selling the site so the Premier League club can build a new stadium there.
Spurs earmarked the site, off Paxton Lane, for a new state-of-the-art £100m stadium after failing to secure a controversial move to the Olympic Stadium.
Anyone with information is urged to call police on 101 or Crimestoppers anonymously on 0800 555 111.