Millions of pounds meant for the arts in Swindon could instead be spent on a congested motorway junction.
The town's borough council wants to take £4.8m from the Swindon Museum and Art Gallery budget and use it to build an extra lane at junction 15 of the M4.
Opposition councillors said the money should be spent on improving air quality.
The money had been earmarked for expanding the museum but the plan fell through due to lack of funding.
Swindon Borough Council, which runs the museum, set aside £5m for that project, but now wants to use most of the money to create a left-turn lane at the roundabout for traffic coming from Swindon on the A419 heading towards London.
The project will cost £17m and the rest of the funding will come from the government and the Swindon and Wiltshire Local Enterprise Partnership.
In 2018, expansion plans for Swindon Museum and Art Gallery fell through when a bid for Heritage Lottery funding was rejected.
Labour group leader Jim Grant said the arts money should be spent on improving the town's air quality.
He said: "I find it incredible that, in light of a recent report which said that 5% of all deaths in Swindon were related to poor air quality, the council has chosen to divert £5m from the art gallery into works on junction 15."
Gary Sumner, Conservative member for strategic planning, said the motorway works would go some way to addressing the issue.
"Congestion on a daily basis on the A419/M4 is a brake on growth, housing delivery and a massive contributor to vehicle emissions.
"The investment in junction 15 is a way of improving air quality, reducing congestion and supporting the carefully planned growth of our town in the decades ahead."
He added the council, which is due to discuss the plans at a later meeting, was still committed to developing the museum as part of a new cultural quarter in Swindon.