In comments described by No campaign leader Alistair Darling as "devastating" for the First Minister, the industrialist and philanthropist said the Scottish Government had over-estimated reserves of oil left under the North Sea by between 45 and 60 per cent. He suggested there were about 15 billion to 16.5 billion barrels of recoverable oil left,
Energy Minister Fergus Ewing said the oil industry had projected that up to 24 billion barrels remained in the area. He added: "There is potentially as much still to come in value terms as has already been extracted."
Sir Ian also warned that, after decades as an oil exporter, Scotland could soon find itself importing energy resources from south of the Border, after the discovery of shale gas in England.
The intervention, Sir Ian's first in the debate, will spark a barrage of questions at Holyrood today over the First Minister's claim that oil revenues would help an independent Scotland increase public spending.
On the Scottish Parliament's last day of business before the referendum, MSPs debated the nation's future after a highly charged First Minister's Questions.
Considering the economic case for independence, Sir Ian, who retired two years ago as chairman of Wood Group which is based in Aberdeen, said the arguments were "quite heavily stacked" towards the conclusion that "our grandchildren and great-grandchildren would actually be better off in the UK".
In an interview with industry website Energy Voice, he stressed he had no political allegiance and said he was intervening for the sake of his grandchildren and future generations of Scots.
He said: "This is a vote not about us or even the next generation, it's 300 years down the road.
"We must take account of the long term, and long-term we'll not have significant offshore oil and gas reserves and that will have a huge impact.
"Young voters right now should just be aware by the time they are middle-aged we are going to see a real run-down, not just in the amount of oil and gas being produced but all the implications from that, the jobs, economic prosperity, public services.
"That run-down will start in 2030, which is only 15 years away, so it's really not right to focus the debate on the short term.
"We really need to think about the medium-to-long term."
Since its discovery there in the 1970s, the North Sea has produced 43 billion barrels of oil.
Based on his recent report for the UK Government, which recommended ways of maximising the extraction of North Sea reserves, Sir Ian said "the best outcome" would be to produce another 15 to 16.5 billion barrels of oil.
Questioning the Scottish Government's estimate of 24 billion barrels - based on the highest of a range of industry predictions - he added: "I believe the Scottish Government's central projection is now between 45 per cent and 60 per cent too high. That's very significant."
Production was due to fall to just 250,000 barrels a day, one-sixth of its present level, by 2050, he said, while major new discoveries were unlikely.
He also said he believed the Scottish Government's prediction for oil taxes - £7 billion a year over the next few years - were £2 billion a year too high.
He warned Mr Salmond's plan to save money into an oil fund "doesn't seem to be realistic" and said lengthy and "extremely difficult" negotiations with the rest of the UK after a Yes vote would create uncertainty and "damage the industry quite badly".
He said Scotland had "disappointing" shale-gas reserves compared with the "massive" quantities found down south.
"It's actually very ironic because in the last 40 years the Scottish offshore basin has made a huge contribution to the UK in terms of energy and oil and gas but we could find ourselves leaving the UK at a time when across the border they have huge energy reserves and in 20, 30 or 40 years' time we'll be importing energy from the rest of the UK and paying for it and affecting our balance of payments."
Better Together leader and former Labour chancellor Alistair Darling said what Sir Ian says "fatally undermines what Alex Salmond has been saying about oil and completely vindicates our analysis."
He added: "Sir Ian Wood's comments blow apart Alex Salmond's plans for funding schools and hospitals. It is devastating for his ridiculous claims on pensions and on jobs."
During FMQs, Salmond accepted Sir Ian was "an authority on North Sea oil and gas" but added that he was "not the only authority of course".
While he said "what Ian Wood has to say on matters should be considered very carefully indeed", Mr Salmond insisted: "The figure I have often quoted of up to 24 billion barrels of oil and gas equivalent remaining in the North Sea is not a Scottish Government figure at source, it is a figure that the industry produced, and I think is a robust figure.
"It shows the extraordinary potential that remains in the waters around Scotland if the policies are pursued and the stewardship is correct to make sure that these resources work for the Scottish people".
He recalled that in 2012 Sir Ian had stated: "My headline message for the youth of today - get involved, the north sea oil industry will see you through your lifetime."
The First Minister insisted: "2050 is not the limit of the oil industry, it will go on long beyond that."
Business group N-56, which is funded by SNP donor and Yes Scotland board member Dan MacDonald, has said oil revenues could be six times higher than the independent Office for Budget Responsibility (OBR) has estimated.
Mr Ewing said: "This is ultimately a debate about exactly how big Scotland's remaining oil reserves are, and most countries are not nearly lucky enough to be in that fortunate position.
"Sir Ian's estimates would appear to be based on projections which only go as far as the year 2050, but which are themselves substantially higher than those of the OBR."