No-one was injured in the blaze, but the coach itself was severely damaged as the dismayed day-trippers looked on.
The incident took place at 10.27am yesterday on the westbound carriageway near the Portsbridge roundabout, Hilsea.
It is believed the fire started due to an electrical fault.
Two fire crews from Cosham and Fareham stations put out the blaze by 10.50am.
Cosham fire station watch manager Steve Pearce said: 'It was a fire we think was caused by an electrical fault.
'The coach was taking a party out on a Christmas outing, so it's a shame it happened.
'No-one was hurt, but the older people had to wait on the hard shoulder for a replacement coach, so the wind blew them about a bit.
'That was probably pretty uncomfortable for them.'
Kent-based coach firm Leo's Pride was not available to comment.
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