Former defence secretary Geoff Hoon told the inquiry the Treasury, under Mr Brown as Chancellor, failed to fund the forces properly in the years before the conflict and then cut their budget after the invasion.

Orders for vital new equipment – including additional helicopters which could have been used in the current conflict in Afghanistan – had to be cancelled as a result of the Treasury’s cost-cutting measures.

“I suppose it’s reasonable to assume that by now, had that budget been spent in the way that we thought we should spend it, then those helicopters would probably be coming into service any time now,” he said.

Liberal Democrat foreign affairs spokesman Edward Davey said it was now essential that Mr Brown – who is not due to give evidence until the summer – appeared before the country went to the polls.

“The Prime Minister should appear before the Iraq Inquiry before the election to give voters an informed choice. Instead we are being left with a huge Gordon Brown-shaped hole in the evidence,” he said.

Mr Hoon said the there had been a strong belief within the MoD that the Treasury had never properly funded the strategic defence review (SDR) of 1998.

Although the SDR was supposed to rectify the historic underfunding of the forces dating back to the end of the Cold War, he said that he was rebuffed when he asked for more money.

Meanwhile, more than 3,000 members of the public applied for seats to see Tony Blair give evidence to the inquiry.

Some 28 families of personnel who died in the operation asked to watch the former PM answer questions live, although six have not been granted places.