The Department for Transport and HS2 Ltd should have raised serious concerns about the methodology in a report claiming the high-speed rail project would bring £15bn in additional benefits to the UK, one of HS2's former advisers has told MPs.

A panel of academic experts told the Treasury select committee that a KMPG report into HS2's regional economic impacts published in September overstated the benefits by six to eight times.

The findings, widely cited by the government – including in the new strategic case for HS2 published last week – used a procedure that was "essentially made up", said Henry Overman, professor of economic geography at the LSE.

Overman, who was an adviser to HS2 Ltd until 2012, said: "At some point HS2, DfT and the peer review should have pointed out that parts of the report were problematic … I don't understand why the £15bn figure was allowed to go out."

He said that even the reduced figure should not be presented as additional to the benefit cost ratio with wider economic benefits published by the DfT, which was revised downward to 2.3:1 last week.

Dan Graham, professor of statistical modelling at Imperial College, told the committee he shared that view. He said of the KPMG work: "I don't think the statistical work is reliable."

Earlier, the KMPG partners who produced the report defended their work as robust as MPs queried the scale and viability of the report, which was produced over four months for a total fee of £242,000.

Committee chair Andrew Tyrie asked: "You don't normally do work of this scale for a couple of hundred thousand do you?"

KPMG's Richard Threlfall replied: "We didn't quite anticipate the degree of debate the report would create."

Tyrie asked KPMG to explain a line in the report admitting: "We recognise that this approach does not have a firm statistical foundation."

The consultants said they had been transparent about their workings. KPMG's Lewis Atter said he believed the work was a "reasonable, probably conservative approach". He said: "There is no perfect way of answering the exam question we are posed."

He said that they stood by the £15bn figure as a forecast, despite a disclaimer that the findings should be regarded only as an estimate. Atter said it was a standard disclaimer.

The KPMG pair denied cherrypicking figures, after a Newsnight freedom of information request showed that cities and towns which would lose out economically from the creation of the HS2 network were omitted from a table in the report which showed only potential gains.

Threlfall pointed to a map included in the report marking with red dots places that would be negatively affected in 2037. He said he thought that a map was more illustrative than a table.

Asked their opinion of the HS2 project, Threlfall said: "Based on the work we've done HS2 is a good thing and we should get on with it."

Atter, who said he started in the sceptical camp, said he now believed HS2 should be built, especially in the context of the wider transport and infrastructure spending ahead.

He said: "The question is not is it the very best thing to do, but would you find space for it in your £1.2tn [spending] over 20 years."

Meanwhile, the transport secretary, Patrick McLoughlin, was speaking at a supply chain conference in Birmingham attended by over 800 companies looking to win contracts totalling more than £10bn.

McLoughlin said businesses from across the UK could be part of building HS2, saying the project would bring jobs and growth to communities across the country for generations to come.

The HS2 network linking London with Birmingham, Manchester and Leeds is scheduled for full completion in 2033 at a cost of £42.6bn, including £14bn in contingencies.