Fire fighters were called to the blaze on Russell Street, at 10.42am yesterday (Sunday).
Crews arrived to find a fire in the stairwell of the property. Wearing breathing apparatus, firefighters used a hose reel jet to extinguish the fire, followed by a positive pressure ventilation fan to clear the smoke.
There were no reported casualties.
Crews returned to their stations by 11.50am.
The cause of the fire was believed to be deliberate. Anyone with information should contact police by visiting www.cambs.police.uk to submit an online report or call Crimestoppers on 0800 555111.