The Welsh Government has been accused of pushing ahead with plans for a £100m automotive park in Ebbw Vale before finalising the business plan.
Conservative leader Andrew RT Davies called it a "cynical ploy" to soften the blow of refusing to back the Circuit of Wales project.
Mr Davies claimed it was "another Labour promise waiting to be broken".
A Welsh Government spokeswoman accused the Conservatives of "talking the plans down".
Economy Secretary Ken Skates has said he expects to see "shovels in the ground" by next March.
The automotive park project, which has an ambition to create 1,500 jobs, was announced in June when Mr Skates rejected support for the £433m Circuit of Wales.
Land is currently being identified for the park, which would develop automotive businesses with £10m a year over 10 years.
A cluster of such firms had already been included in the Circuit of Wales plans.
In response to a written question from the Conservative leader, Mr Skates said: "We will publish a full business case in relation to the automotive technology park later in the year, which will of course be informed by relevant expertise and assessments."
Mr Davies said this cast doubt on a plan he claimed had been plucked "out of thin air".
"The Welsh Government's decision to pull the plug on the Circuit of Wales after seven years of empty promises was a huge blow, but the new plans for an automotive park are yet another Labour promise waiting to be broken," he said.
"We don't even have a finalised business case yet.
"How can the Cabinet Secretary claim that the biggest lesson of this whole debacle was the importance of proper due diligence, while he is simultaneously committing £100m to a scheme that no one had even heard of until last month?
"We need to know that proper due diligence is still going to be followed in this case - and the people of Ebbw Vale will want to know what will happen to these promises if the numbers don't stack up."
A Welsh Government spokeswoman said: "As the Conservatives already know, extensive external and internal due diligence around the Circuit of Wales project made clear that the main benefits to the local economy would come from businesses clustering in Blaenau Gwent, rather than directly from the race track."
While the Conservatives had been "talking the plans down", Mr Skates had been meeting businesses, academics and local politicians to make the plan a success, she added.
"There is strong and collective ambition between Welsh Government and its partners to shape and deliver a project that can genuinely regenerate the local economy and create the high quality jobs the area needs," she said.
"We will publish a business case later in the year that draws upon specialist advice and reflects the views emerging from our partners."