Three mobile homes and a van were alight near Claybank Road at around 10.30pm with fears the fire could spread.
The front of heavy goods garage Adams Morey LTD, on Burrfields Road, was also on fire.
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‘The mobile homes had gas cylinders in them resulting in explosions,’ Cosham watch manager Craig Sadler said.
‘I made the decision to evacuate and close Burrfields bridge and for people to shut windows and doors.
‘Black smoke was coming across the bridge, I urged for trains to run with caution.’
The fire was out in around an hour with no one hurt.
Four pumps, six breathing apparatus, three jets and two hose reels were used to put out the fire.
Cosham fire crews were joined by Portchester, Southsea and Havant.
An Adams Morey employee, who wished to remain anonymous, told The News three campervans were damaged in the fire.
'There was also damage to the outside of one of the buildings,' he said.
'But there was no internal damage and no one was at risk.
'We think it may have been caused by an electrical fault. The fire brigade came in the night and sorted it all out.'
The fire was not being treated as suspicion with investigations set to be carried out to find out what caused it.