An early cost estimate MPs had when they were approving the HS2 high-speed rail project was "enormously wrong", a former HS2 boss has told BBC Panorama.
Doug Thornton said the costing underestimated the value of many properties HS2 needed to purchase along the proposed route and thousands more had not been budgeted for.
Mr Thornton said the figure MPs saw was hundreds of millions of pounds too low. He was later dismissed.
HS2 rejects claims MPs were misled.
It says it followed the correct process to keep Parliament updated on budgetary changes.
Another former senior HS2 insider agreed that the company had had estimates that were higher than the early costing that MPs had at the time.
HS2 is the biggest infrastructure project in Europe. The government says it will cost £56bn, with the first phase from London to Birmingham coming in at £27bn.
The spending watchdog, the National Audit Office, investigated the land and property programme earlier this year.
It found estimated property costs did increase significantly but concluded HS2 was not required to provide its updated estimates to MPs.
However, Mr Thornton said he refused to use figures he believed were misleading.
"I have never seen anything like it", he told the BBC.
"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," he said.
Mr Thornton was subsequently sacked by HS2.
The Sunday Times previously reported that it had seen documents in which Mr Thornton said HS2's official budget for buying land and buildings was based on "rudimentary map-based analysis by interns".
These new claims come amid recent turmoil at the company responsible for the project.
When the joint chairman of HS2 and the troubled Crossrail project, Sir Terry Morgan, resigned earlier this month he warned of possible cost issues ahead for HS2, talking of the "challenge inside the project" to stay on time and on budget.
The railway company is involved in the biggest land and property acquisition programme since World War Two, buying a total of 70 sq km of land.
Panorama has spoken to a number of people whose properties are in the path of the high-speed railway, who have complained of low valuations and delays in compensation after finding out their homes and premises were needed by HS2.
Ron Ryall's Elizabethan home is in the path of the viaduct that will carry the railway in Buckinghamshire. He said HS2 had offered him far less than the property is worth.
Meanwhile, Paul Tropman who owns Wood Waste, a wood recycling business on the outskirts of Birmingham, said he had been left hundreds of thousands pounds out of pocket after his business was forced to relocate.
"I think they've overspent and the only way they can get their money back is by robbing the likes of us," he told the BBC.
HS2 is still working to settle their cases. Chief executive Mark Thurston said the company always paid a fair price and has only had to use its compulsory purchase powers in a small number of cases.
Mr Thurston did recently acknowledge a cost gap within the project as a whole and that he is in talks with contractors to reduce costs.
But he told Panorama: "I'm not worried about overspending, I'm confident we've got a budget we can stand by."
You can watch Panorama: HS2 - Going off the Rails? on Monday 17 December at 19.30 on BBC One, or on BBC iPlayer afterwards.