A former HS2 boss has said MPs’ early cost estimate for the controversial rail project was “enormously wrong”.
He said the figure MPs were given was hundreds of millions of pounds too low. Mr Thornton was later sacked from his job. HS2 has rejected claims that MPs were misled.
Another former HS2 insider told the BBC the company had estimates that were higher than the early costing given to MPs at the time.
The first phase of HS2, which the government says will cost £56 billion, will run between London and Birmingham and cost £27 billion.
The National Audit Office investigated the company earlier this year and found estimated property costs along the route did increase dramatically, but that HS2 was not required to give this information to MPs.
But Mr Thornton told the BBC he refused to use figures he thought were misleading.
“I have never seen anything like it,” he said.
“I was absolutely appalled… that numbers could be advanced in such a loose and slapdash fashion.
“There was a gap of almost 100% in terms of the wrong numbers of properties that the organisation had not budgeted for.
“We started to talk about it to our finance team, we talked about it to HR, we talked about it to line managers, so it was there, we were calling it out.”
The Sunday Times has previously reported that it had seen documents in which Mr Thornton said the company’s budget for buying properties was based on “rudimentary map-based analysis by interns”.
Sir Terry Morgan, the joint chairman of HS2 and the Crossrail project, resigned earlier this month.
He warned of the “challenge inside the project” to keep HS2 on time and under budget.
A number of people told Panorama their properties had been vastly undervalued by HS2.
Chief executive Mark Thurston said HS2 paid a fair price for properties.
“I’m not worried about overspending,” he told the BBC. “I’m confident we’ve got a budget we can stand by.”
Panorama: HS2 – Going off the Rails? is broadcast on BBC One on Monday at 7.30pm.