A taskforce set up to protect jobs at Ford's Bridgend plant did not meet for eight months before the factory's closure was announced.
Plaid Cymru leader Adam Price called it "staggering", and said it was a "dereliction of duty" by the Welsh Government with "a party asleep at the wheel of governance".
Ford announced with the loss of 1,700 jobs.
The Welsh Government said it "strained every sinew" to keep the plant open.
A spokesman added the comments from Mr Price did not reflect the time and effort put into supporting workers.
Set up in 2017 by officials in Cardiff Bay, the working group was tasked with helping to protect the plant's future.
But it did not meet between 9 July 2018 and 18 March 2019.
"This raises serious questions on what exactly the government were doing during this crucial period where the future of the plant was still hanging in the balance," Mr Price said.
The group, which was chaired by Welsh Government ministers and officials, had representatives from Ford Europe, Ford UK, the Wales Office, the UK government's Department for Business, Energy and Industrial Strategy, the Welsh Automotive Forum and Unite the Union.
Mr Price added: "The Welsh Government knew back in January this year that the plant could face a potential loss of over 1,000 jobs.
"It is therefore inconceivable that the Labour Welsh Government taskforce failed to meet with Ford until two months later."
The taskforce met five times in the three months after it was set up, but only once in the 11 months before the plant's closure was announced.
Mr Price received the information after submitting a written question to Welsh Government Economy Minister, Ken Skates.
Plaid Cymru has organised a public meeting Monday evening for workers and companies in the local supply chain at 18:00 BST at Vale Cricket Club, Corntown.
The Welsh Government spokesman said: "This is a cynical attack that in no way reflects the time and effort put into supporting the Ford Bridgend plant and its workforce by the Welsh Government.
"Welsh ministers and their officials have met with representatives from Ford very regularly over the past two years to explore internal commercial options for the site.
"The Welsh Government has strained every sinew to keep the Bridgend plant open - including making significant investment in the site and its workforce - and makes no apology whatsoever for continuing to do so in order to keep good quality jobs and production in the area."
He added the task force was "just one of a number of ways" the government worked with the company to try and safeguard jobs.
A Unite union spokesman said: "It is the decisions and actions of Ford itself that have put thousands of jobs at Ford and in the wider automotive supply chain at risk.
"Our members are not interested in political point-scoring, it's an unwanted distraction.
"What they expect is for politicians from all parties to join them in calling upon Ford to think again and reverse its devastating decision to close the Bridgend plant".