The party's Assembly Group Leader, Raymond McCartney lodged his concern with the Speaker's Office on a DUP Amendment to a Sinn Fein motion.
It was calling on the Ulster Unionist Party (UUP) Department of Regional Development Minister (DRD) to give an assurance that there will be no dilution of the project or delay in its completion.
Raymond McCartney said: "Sinn Fein secured a debate on the Aughnacloy/Derry Dual Carriageway in order that we could impress on the DRD Minister the crucial importance of this project to the economic regeneration of the North West.
"I am disappointed therefore that the DUP 'seen fit' to put forward an Amendment that would dilute the importance of this project going forward as originally envisaged.
"I would even at this late hour urge the DUP to consider the detrimental affect it will have on the North West, a region that has been starved of infrastructural investment for decades, if this development does not go ahead as planned.
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"Only last week the Irish government once more confirmed its original financial commitment to its construction," he said.
Last month, the future of the major cross-border road project seemed in doubt after the Stormont DRD Minister, Danny Kennedy said he would look again at plans for the western corridor development.
"I will be looking closely at the level of proposed investment on roads such as the A5," said the newly appointed Ulster Unionist Stormont Executive member.
He said that his emphasis would be on backing sustainable transport and improved road and water infrastructure to support economic growth and social wellbeing.
The UUP Minister has taken a much more guarded view on the A5 than his Sinn Fein predecessor, Conor Murphy, who - despite ordering an Inquiry - was fully committed to the high-budget construction project.
UU Leader Tom Elliot is on record as saying that while the prospect of an upgrade was welcome, that would not be at any cost.
"Opposition to these plans is based on a number of factors, from the environmental and personal impact of the build and of increased traffic, to the financial cost. These are all justified concerns and must be given a fair hearing," he said.
These worries led to Conor Murphy announcing earlier this year that a Public Inquiry would indeed be held into the proposed A5 Western Transport Corridor dualling project.
He said the formal consultation period associated with the project, which ran from 15 November 2010 until 21 January, attracted over 2,000 objections.
See: Stormont Minister Signals A5 Slow-down
(BMcC/GK)
