A high speed train service between Birmingham and London is far from a done deal and could be thrown off course by further deterioration in western economies, the chairman of Network Rail has warned.

Rick Haythornthwaite said that while all of the main political parties in this country were backing the £17 billion HS2 project, sentiment among MPs and Ministers could easily change if the public became concerned about the cost to the public purse at a time of austerity.

Speaking at a CBI lunch in Birmingham, Mr Haythornthwaite said the business community had to “pick up the mantle” and speak out strongly in favour of high speed rail, which he said would deliver growth at a time when other methods of boosting the economy were not working.

He told his audience: “We cannot take political sentiment in favour of HS2 for granted. This is a vital moment for high speed rail.”

He added that at times the “silence from the business community is deafening” and urged companies to make their voices heard.

However, Mr Haythornthwaite’s comments came as it became clear that business chiefs are split over the value of high speed rail. A survey of more than 1,000 directors revealed many consider the planned high speed rail line between Birmingham and London is poor value for money.

Researchers for the Institute of Directors (IoD) found that HS2 is a long way down the list of transport priorities for many business people.

The survey, which asked directors for their priorities for new investment as well as their views on the current state of Britain’s transport infrastructure, also revealed that business leaders think that improvements to existing roads are more important than new roads.

Simon Walker, director general of the IoD, said: “Our survey shows that directors want to see improvements to existing roads and rail services above all. In straightened times, choices will have to be made between some improvements to existing rail lines and a new high speed line, and we want to see the Government take the business view seriously.”

The IoD survey follows a study by the Institute of Economic Affairs earlier this year which alleged that the high sped plan was based on “flawed economics” and said claims that the service would benefit economies in the Midlands were simply false.

"Birmingham City Council has been campaigning strongly for the high speed link, with the help of research by KPMG which suggests the project will create 22,000 jobs in the West Midlands metropolitan area and lead to a £300 average increase in pay for every working person.

The council believes the 250mph trains will make the Midlands far more attractive for firms wishing to relocate from London and the South-east, a claim supported by Mr Haythornthwaite who said reduced journey times between the two cities would change the way Birmingham thought about London.

He added: “HS2 means that Birmingham will be seen as being in the same zone as London. There will be a real shift in mindset.”

Mr Haythornthwaite set out a scenario where the Government might still be forced to backtrack on HS2 if public opinion began to drive the agenda.

He said: “The Government is sceptical about policies that appear to be unpopular. The argument against large capital projects is understandable when the global economy is behaving the way it is and budgets are severely constrained.”

He pointed out that most of the UK rail infrastructure remained “rooted in Victorian times”, adding that HS2 was essential to free up capacity on the West Coast Main Line between Birmingham and London, which would be unable to take any additional passengers after 2025.

Mr Haythornthwaite said: “I don’t just mean there will be overcrowded trains where you might be standing. Peak trains will be so full that passengers will be turned away. There will be no standing or seating space left.”

He was forced to admit, however, that Network Rail plans to spend almost nothing on new infrastructure in the Midlands between 2014 and 2019.

Less than one per cent of the organisation’s budget has been allotted to the Midland region. Most of the money will be swallowed up by rail electrification plans in Manchester and Liverpool and a programme to transform the East Coast Main Line.

Anyone who thought bids for cash from other regions would be accepted was being “delusional”, Mr Haythornthwaite added.

Strategic long-term planning was required to prevent high-spending but necessary projects being dropped under pressure from “nimbys”.