Opponents of a high speed rail line between London and Birmingham slammed the case for a new service as “fatally flawed” after it emerged that some inter-city trains on the West Coast Main Line are only half full – even at peak times.

Ministers argue a new high speed network linking the West Midlands with the capital, and eventually extending to the north of England and Scotland, is essential because the West Coast Main Line is running out of capacity.

But a new study found that long distance trains leaving London in peak time were only 56 per cent full, on average.

Researchers counted the number of passengers boarding West Coast Main Line trains at London Euston between 4.30pm and 7pm, on three days – a Tuesday and two consecutive Thursdays.

The trains were heading to a range of destinations including Liverpool, Manchester, Birmingham and Glasgow.

They found that trains to Manchester and Liverpool were just 45 per cent full on average, when the number of passengers was compared to the number of seats on the train.

The 5.03 to Birmingham was 75 per cent full on average, while the 6.03 to Birmingham was 78 per cent full. The only service included in the study which was regularly over capacity was the 6.43 to Wolverhampton, which had an average of 477 passengers to just 447 seats.

Regular travellers crammed into standard class carriages may be surprised to hear that the trains had seats to spare, but the researchers looked at capacity across the entire train – including first class carriages which had room to spare.

They said in their report: “If there is crowding in standard class this could potentially be dealt with through reconfiguring the carriage mix.”

The research was funded by HS2 Action Alliance Ltd, a campaigning body opposed to the planned new high speed line, and conducted by independent researchers Customer Research Technology Ltd.

Speaking in the House of Commons, David Cameron said the Government remained committed to building the new line, known as HS2, despite ministers announcing that their response to a public consultation, originally due in December, had been delayed until January.

The Prime Minister said: “Clearly the country has a choice. Because the West Coast Main Line is as congested as it is, we need either to replace it with a traditional line or a high speed line.

“It is well known the Government’s view is that a high speed line is the right answer. That’s why this consultation has been conducted and I think it’s not just good for people who want to use the West Coast Main Line but also will be a successful regional policy in linking up our cities, shrinking the size of our country and making sure that all parts of our country can enjoy economic prosperity and growth.”

Business leaders across Birmingham are continuing to campaign in favour of a high speed line, saying it is essential to cut congestion on the rail network.

Simon Topman, managing director of ACME Whistles, based in Birmingham’s Jewellery Quarter, said: “I have had to stand all the way on many occasions.

“And if demand for rail services continues to grow as the predictions say then it is only going to get worse. We are going to have gridlock on the rail network.

“We do need another line and the impact on the environment will be just as great if it is a conventional line as if it is high speed.”

He said he was struck by the contrast on visits abroad between modern high speed lines in many foreign countries and the UK rail network.

“It’s not good enough to be quaint and second rate any more. We need to be modern Britain, we need to be cutting edge.”

“Quicker connectivity and reducing congestion will be good for my business.”

ACME Whistles employs 65 people and designs and manufactures professional whistles, including products used in international sporting events, by police forces and by safety organisations such as lifeguards.