THE workplace productivity behind Alex Salmond's promise of a £1000 independence bonus for every Scot remains "an area of uncertainty", his own economic experts have warned.

The assessment is contained in the latest annual report by the chairman of the First Minister's Council of Economic Advisers (CEA).

The Government last night said only short-term productivity was uncertain, whereas independence could produce permanent change, but opposition parties said Mr Salmond had been caught peddling "fantasy". The First Minister said last week that higher productivity, higher employment and higher immigration after a Yes vote would result in an extra £5 billion in tax revenues after 15 years, an "independence bonus" worth £1,000 per person.

The assertion was also used to counter warnings by the Institute of Fiscal Studies yesterday that an independent Scotland faces higher tax hikes or deeper spending cuts to pay for the SNP Government's flagship "giveaways" on childcare and pensions.

The CEA chairman's report, which ministers released online without publicity two days after the £1,000 bonus offer, indicated productivity trends could not be taken for granted.

It said "the outlook for the labour market and earnings, both in Scotland and the UK, depends on future productivity trends, which remain an area of uncertainty".

Labour finance spokesman Iain Gray said: "Every day we see independent analysts telling us these figures do not add up."

A Scottish Government spokeswoman said: "The text quoted from the Chair's Report relates to short-term changes in the Scottish and UK labour markets. The First Minister was referring to permanent improvements in Scotland's productivity growth rate, which can be supported by access to the economic levers available under independence."