Firefighters are tackling the blaze at the former Barclays bank in Shirley Road, next to Santo Lounge.
Around 100 people were watching the fire on the pavement next to Lidl, which is opposite the bank.
Posting on X, Bluestar said: “Significant disruption to B* 4, 7, 17, 18 & 19 because of a road closure.
“We currently have 10 buses across the 5 routes stuck in Shirley, so there may be large gaps in service while we work to free the buses & reposition them to their scheduled times. Apologies.”
Another post added: “Due to bus still stuck in road closure, Shirley High street due to Fire. The Romsey B*4 from 17:55 – West Quay is not running. Apologies.”
