THE cost of Scotland's fledgling tax collection system has risen by more than 20% since the publication of the SNP's White Paper, prompting questions about the robustness of other predictions in Alex Salmond's blueprint for independence.

Revenue Scotland, which will initially collect just two devolved taxes from 2015, had been due to cost £16.7 million during its first five years.

The figure was included in the 670-page White Paper on independence published on November 26.

However, ministers have revised the price up by £3.5m, or 21%, to £20.2m after identifying the need for new activity and investment.

The higher figure is buried in background notes to the Revenue Scotland and Tax Powers Bill that was introduced to Parliament by Finance Secretary John Swinney on Thursday.

Unionist parties said the higher figure raised questions about the price tag for collecting all of Scotland's taxes under independence, as well as the validity of other White Paper promises.

Earlier this year, a leaked memo written by Swinney warned of "significant" costs attached to creating and running some state institutions under independence, with tax administration estimated at between £575m and £625m, more than double Scotland's current contribution towards Her Majesty's Revenue and Customs (HMRC).

The anti-independence group Better Together said the White Paper had fallen apart.

The original cost estimate for Revenue Scotland was issued by Swinney in June 2012.

The new body will collect two taxes that are being wholly devolved under Westminster's 2012 Scotland Act - the Land Buildings Transaction Tax (LBBT), which replaces Stamp Duty on property sales, and the Scottish Landfill Tax, which replaces the UK version.

If Scotland were to become independent, Revenue Scotland would expand to collect all taxes. Swinney told parliament the initial, narrow set-up and running costs of Revenue Scotland from July 2013 to March 2020 would be £16.7m.

The figure was repeated in the White Paper last month along with a prediction it would be 25% less than HMRC would charge for a like-for-like service.

"Building on that, we will create a tax system in Scotland that is simpler and costs less to administer than the current UK system," the accompanying text said.

However, the financial memorandum to the bill creating Revenue Scotland says the £16.7m has been "reviewed and revised" as "additional costs have been identified in relation to three areas not included in previous cost estimates".

These include £1.5m for extra IT investment, £730,000 for Scottish Tax Tribunals, and an extra £1.05m for the Scottish Environment Protection Agency (Sepa), which will help administer the landfill tax to investigate and bill those who are found to be responsible for illegal dumping.

Revenue Scotland will also spend an initial £230,000 on investigation and compliance work in its first year, a sum that may be repeated, potentially adding another £1m to the bill.

A spokesman for Better Together said: "The SNP's White Paper is barely a month old and already it has fallen apart.

"First their assertions on the EU and currency crumbled and now it turns out that their quoted figure on the cost of Revenue Scotland is totally inaccurate.

"The entire document is lacking in any kind of credibility, which is no real surprise as Alex Salmond has based his entire case for breaking up the UK on uncosted promises, bald assertions and dodgy statistics."

A Scottish Government spokesman said: "The White Paper correctly sets out the basic set-up and running costs of Revenue Scotland taking on tax powers currently administered by HMRC.

"The financial memorandum sets out additional plans to enhance Revenue Scotland's tax collection capacity, over and above that currently provided by HMRC.

"These include an IT system to support additional work on tax compliance, and new measures to address illegal landfill dumping."