Spanish giant FCC and local civil engineer Alun Griffiths (Contractors) were due to start on site in Ebbw Vale, Blaenau Gwent late last year.

But planning wrangles have delayed the start and bosses at Silverstone racetrack have now lodged an objection to any state aid for the project.

Silverstone leaders have written to David Cameron seeking assurances that no public money will be pumped into the scheme.

It is believed developers behind the Welsh project – Heads of the Valleys Development Company – are asking for £50m from the Welsh and UK governments.

Silverstone said: “Silverstone Circuits Limited can confirm that it has written to the Prime Minister, requesting confirmation that UK government is not considering financial support for the Ebbw Vale Circuit of Wales project.

“Silverstone has been advised that an injection of funds by the Welsh and/or UK governments to the Circuit of Wales project could also amount to illegal State aid.

“To be clear, Silverstone has no objection to competition between UK circuits, but it has to be a level playing field.

“It is Silverstone’s view that the Circuit of Wales project is unviable.

“Silverstone reiterates that it is sympathetic to the economic situation in Ebbw Vale, but motor sport at circuit level is not particularly profitable and is often loss making.

“Many UK circuits are under-utilised and struggle for revenue. It is unrealistic to suggest Circuit of Wales will be the exception.”

The Circuit of Wales would host all motor sports events except Formula One.

Michael Carrick CEO of the Heads of the Valleys Development Company, said: “We are disappointed to have heard of the content of this letter, it sounds like Silverstone is resistant to investment within the automotive sector which is vital to its continued growth and development in the UK.

“The suggestion of ‘ illegal state aid’ made by Silverstone also highlights they do not understand the basic process of applying for state funding of this nature.”

The developer added: “There is significant support and commitment from investors to make this project a reality, and, at an appropriate time, we will be announcing our partners and investors to deliver what will be the largest investment in UK automotive infrastructure for 50 years.”

Alun Griffiths and FCC held a “meet the buyer” even late last year for the project which was attended by 400 local suppliers and subcontractors.