The trees were removed from the Market Place in Melksham, Wiltshire, as part of a planned redevelopment.
Volunteer Kathy Iles said she "stood in the town hall and cried" after seeing the felled trees while walking "the route for a last check".
Wiltshire Council apologised and said hoped it "doesn't affect the judging".
Ms Iles said while walking the route on Wednesday, to her "horror" she saw a team of contractors in the Market Place cutting down trees.
She said judges did "not just look at the blooms but took everything into account".
"It's about community participation, about cleanliness of the town," she said.
"These trees are due for removal as part of the Market Place redevelopment but they're not supposed to be coming down this week.
"And definitely not the night before judging. I was just absolutely horrified. I rushed across the road and tried to get them to stop but they're there to do the work aren't they?
"They haven't even finished the job. There's a tree stump left standing in the Market Place. The other tree is half-done and it's got Christmas lights dangling from it. It just looks awful."
A Wiltshire Council spokesman said: "These trees were always planned to be removed as part of ongoing investment in Melksham. They are in a poor condition and had to be removed as part of this work."
They said the authority had been doing "all we can" to make the Market Place look "as nice as it can in the circumstances".
Britain in Bloom is a nationwide community gardening campaign which aims "to transform cities, towns and villages".
It was set up by the Royal Horticultural Society in 1964.
Volunteers preparing for their town's Britain in Bloom bid say they were "absolutely horrified" when the council cut down trees the day before judging.