WHEN it comes to North Sea oil, Sir Ian Wood stands head and shoulders above the rest.

Over 48 years he built-up one of Scotland's most successful companies - the Wood Group - with 46,000 employees and a turnover of £7billion.

Quite rightly, Sir Ian is recognised the world over as a pre-eminent authority on oil and gas. As those in the industry will tell you, when Sir Ian speaks you better sit up and take notice.

Last week he did exactly that, warning that Alex Salmond has over-estimated remaining oil reserves by 45 to 60% and that "offshore oil and gas cannot figure significantly in Scotland's medium-term economic calculations".

Sir Ian has no dog in this fight.

He has no political allegiance and he isn't a member of any referendum campaign.

Rather, he is an expert - a leading businessman and successful entrepreneur, who has decided to speak out because he wants the best for future generations of Scots.

If, as Sir Ian estimates, it will only be 15 years before oil reserves begin to dry up, then the decline in production would have a severe impact on the economy of an independent Scotland.

To plug such a gap, the impartial Institute for Fiscal Studies calculated that a separate Scotland would have to increase the basic rate of income tax by 10%, or VAT by eight.

Faced with such a choice, it's no wonder that Sir Ian concluded "the case is heavily weighted towards Scotland remaining in the UK".

It's just common sense that a larger, more diverse economy is better placed to deal with volatility in either oil price or production.

We know that the Scottish Government has consistently overestimated oil revenues - in the financial year just past, SNP ministers expected oil to generate up to £8.3bn but the true figure was around half that.

A £4bn gap is close to the entire education budget for Scotland.

As part of the United Kingdom, such a shortfall is manageable - but in an independent Scotland, we simply couldn't afford for the maths to be this far out.

While Sir Ian knows this only too well, Alex Salmond refuses to acknowledge that North Sea oil can't last forever. Instead of re-hashing the arguments of the 1970s, we need to look long term and plan for the future.

Oil and gas are tremendous assets for the Scottish economy, but by over-inflating their worth, the nationalists risk misleading the people of Scotland.

This is what makes Sir Ian's reluctant intervention all the more timely.

He spoke out not as a hired gun, or on behalf of any political campaign - but in a personal capacity, motivated by the impact the referendum will have "on children, grandchildren and future generations".

His words were considered, powerful and cannot be dismissed. They serve as a clear, unambiguous statement of why Scotland's long term economic prosperity is best secured by remaining part of the United Kingdom.