Three stores have been severely damaged in a blaze which started at about 0400 GMT at Boucher Retail Park in Belfast.
Seventeen appliances and 85 firefighters struggled for several hours to bring the fire at Boucher Crescent under control.
Poundstretcher, Birthdays and JD Sports were worst hit while other nine stores were affected by smoke and water.
A Fire Service investigation team is working with police to establish the cause of the fire.
Assistant Chief Officer Eoin Doyle, however, said it could be several days before it was known how the fire started.
"The fire appears to have started at Poundstretcher. It also spread to the adjacent properties on either side."
He said officers were hampered when part of the gutted Poundstretcher building collapsed.
Poundstretcher has 27 stores throughout Northern Ireland.
Company secretary Martin Collinson said 17 or 18 staff worked in the Boucher Crescent branch.
"Northern Ireland is a hughly important market for us," he said.
"It's one area of the UK where we are over-represented to some extent.
"Because we have a number of other stores in relatively close proximately, we will seek opportunities to re-deploy people where that's possible."
Public Safety Minister Michael McGimpsey praised the work of firefighters tackling the blaze.
"I commend them for the fearless way that, over many hours, they have tackled the fire, contained it and prevented further damage," he said.
"These 85 firefighters have put their lives on the line to fight this blaze and have done a magnificent job. Thankfully at this stage there have been no reported injuries."