Marks & Spencer in Ipswich town centre was closed for most of the day after a carbon monoxide leak was detected.
National Grid was called to the store in Westgate Street at 6.30am to reports of a smell of gas in the basement.
Engineers did some tests and found a “small amount” of carbon monoxide, a National Grid spokeswoman said.
All of the staff were evacuated from the shop and part of the street cordoned off while investigation work took place.
Three neighbouring shops also experienced a power cut around the same time so engineers from UK Power Networks attended.
The electricity was restored at 14.43pm
Suffolk Fire and Rescue Service was called to Marks & Spencer around 8.30am and sent three fire crews from Ipswich East and Princes Street.
Crews found that the leak was caused by an “underground fault” and used positive pressure fans to ventilate the basement.
A small fire was found in electric cable but it was “dug out” and extinguished, a fire spokeswoman said.
The gas was within safe limits at 12.17pm and firefighters continued to monitor the levels for any arising leaks until 3.38pm when the stop was called.
Speaking at the scene, station commander from Suffolk Fire and Rescue Service Steve Rhind said: “The electricity board and gas board were mobilised to a leak this morning. They detected carbon monoxide in the basement. We used breathing apparatus and found high readings.
“We know there’s gas down there and we have now handed things over to the gas and electricity board but we will still have a presence here.
“Carbon monoxide is a poisonous gas, it’s a silent killer, so we can’t have people wandering around in the basement so we are doing safety checks and making sure it’s safe for the general public and staff.
“We will be keeping the shop closed until the readings are clear in the basement.”
A Marks and Spencer spokeswoman said following a floor safety review and permission from the local authority, the shop reopened at 4pm and would stay open until 9pm for late-night Christmas shopping.
She said staff from the Westgate Street store were relocated to other nearby Marks and Spencer stores while it was closed.
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