A decision on whether a new off-road car will be made by Ineos Automotive in Bridgend is expected within days following a board meeting this week.
BBC Wales understands Ford in south Wales is in competition with a site in Portugal to build a new car inspired by the Land Rover Defender.
A UK government letter seen by BBC Wales emphasises "unrivalled corporate tax support".
It also highlights the skills of the local workforce.
Signed by Business Secretary Greg Clark and Welsh Secretary Alun Cairns, the letter commits them "to explore exactly how the UK might support you on this exciting opportunity".
It points to the "rich and unique heritage" the UK has in car production including the Land Rover Defender.
Ford employs 1,850 people at its Bridgend plant, though there have been concerns for its long-term future as it stops producing engines for Jaguar Land Rover in 2020.
There have been problems with industrial relations at the factory and documents previously leaked to BBC Wales showed the cost of producing engines was significantly higher than at Ford's Dagenham plant.
Those supporting Bridgend's bid for the work hope that Ineos has been convinced that the situation has improved at the site.
UK corporation tax has been cut from 20% in 2015 to the current rate of 19% and will be further reduced to 17% by 2020. Corporation tax in Portugal is 21%.
Ineos Automotive's new car aims to replace the Land Rover Defender, which went out of production in 2016.