It is appalling Welsh ministers are spending nearly £20m preparing for a planned M4 relief road around Newport, Labour AM Jenny Rathbone has said.
Construction is not due to begin until 2018 but "assessment work and public engagement" is costing £19.8m.
Economy Minister Edwina Hart defended the spend saying it was "absolutely normal" for such a project.
Opposition and several Labour AMs oppose the £1bn 'black route', which ministers call "vital" for the economy.
The proposals include 15 miles (24km) of new motorway and a 1.5 mile-long (2.5km) bridge across the River Usk.
A major remodelling of junctions 23 and 29 of the M4 is also planned.
Building work on the project is due to begin in spring 2018, but there will be a public inquiry into the plans at the end of 2016.
In March, the Welsh government appointed construction companies Costain, Vinci and Taylor Woodrow to begin development work.
As part of a budget deal with the Liberal Democrats, construction work will not begin until after the 2016 assembly election.
But Cardiff Central AM Jenny Rathbone said she wants Labour's assembly election manifesto to pledge to review the plan, claiming "most" Labour backbench AMs oppose it.
She told BBC Wales: "I'm both astonished and appalled that up to £20m is being spent on an M4 relief road I hope never happens.
"I'm obviously pretty disappointed we're spending up to £20m on engagement and assessing whether or not this motorway should go ahead, when I think that nearly all backbenchers are opposed to this.
"To spend £1bn in government borrowing on a road when we really need to be spending money on improving public transport is very disturbing.
"Everybody's hopeful that we will have a commitment to review the M4 relief road in the manifesto and certainly not to go into the election with a commitment to go ahead and build it.
"I don't think we have that mandate at the moment and we certainly don't want to see that in the manifesto."
The minister behind the scheme, Edwina Hart, is standing down at the election.
She said: "Everything we've spent is absolutely normal, in terms of how you do a big project.
"Roads will always be controversial, but 20 years after you've built them they're not controversial, they're the norm. We've got to look at the M4 in the context of the economy."
Employers' organisation the CBI has warned the project must not lose momentum when Mrs Hart leaves.
While opposition parties differ on their preferred solution to M4 congestion, there has been criticism of Labour's handling of the project.
The Welsh Conservatives' Shadow Transport Minister William Graham said: "We all know that something needs to be done urgently, but the in-fighting in the Labour assembly group appears to be causing even greater confusion."
Plaid Cymru, whose Freedom of Information request revealed that spending on preparations had risen to £20m from a predicted £7m, claimed the Liberal Democrats had been "suckered" by Labour into backing "its £1bn M4 vanity project".
AM Simon Thomas said: "They must be hugely embarrassed that the Labour government has done the dirty on them and boosted spending on preparation work, rather than reining it in."
The Liberal Democrats denied being duped by ministers, saying they knew money would be spent on the project because the Lib Dems demanded a detailed environmental impact study on the scheme be carried out.
Lib Dem AM Eluned Parrott urged the Welsh government to accept that its M4 plans were "doomed" and pursue a cheaper, less environmentally damaging alternative instead.