It was discovered by BBC journalist Sue Charles, who found it abandoned with its doors open, blocking a lay-by, at around 11pm on Saturday when she was driving to Cardiff.
“From a distance, it looked as if it was an accident but as you got closer you could see it was in a hedge and you could see it was a Brexit bus,” Ms Charles said.
“It thought it was very odd that it had been left side-on blocking the layby.”
She added that she went to check if anyone had been injured but found no one inside.
Although Dyfed Powys Police reportedly told Ms Charles they were aware of the bus and had sent someone to investigate it, no official statement has been released on the incident.
A local vehicle recovery company said they were asked to tow the vehicle to Cardiff on Sunday morning.
Although Brexit Party candidate Des Parkinson said the bus broke down last night, Derek Jeffs, a local removal company boss, told Wales Online that he saw the bus stranded on Saturday afternoon.
“It looked as if the bus had turned into the layby but there wasn’t enough room to turn it around. However, the driver couldn’t reverse because the road dipped down,” he said.
“The driver was still in the cab and he didn’t look best pleased.”
The Brexit Party are currently attempting to win their first parliamentary seat in the Brecon and Radnorshire by-election.
The Liberal Democrats are favourites to win the election, with the Brexit Party predicted to challenge the Conservatives for second place.
Voters in the constituency will go to the polls on Thursday.
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