HMRC: Finding Scottish taxpayers 'more complex than predicted'
The tax collection agency HMRC has said work to identify Scottish taxpayers is proving "more complex" than anticipated.
It wants access to NHS records to help identify those eligible to pay the new Scottish rate of income tax (SRIT).
Labour has called on the Scottish and UK governments to "get a serious grip" of the situation. HMRC said the project was "on track for successful delivery".
From April 2016, Holyrood will set income tax rates for the first time.
Only those who spend most of the tax year resident in Scotland will be required to pay the Scottish rate.
HMRC is working to identify these Scottish taxpayers.
In a letter to the Commons public accounts committee, it said: "This has proved more complex than was initially anticipated, which is why the rating for the risk relating to this area of work has increased."
The letter, dated 16 January, was written by HMRC's tax assurance commissioner Edward Troup.
Shadow Scottish Secretary Margaret Curran said: "This letter confirms what Scottish Labour has been saying all along - that HMRC and the Scottish government are struggling to identify all Scottish taxpayers.
"Both our governments need to get a serious grip of this."
The Scottish government said administering the tax was a matter for the UK government and HMRC.
HMRC said it remained "confident" that it would "be able to deliver SRIT and correctly tax Scottish taxpayers from April 2016".
It has also revised the estimated cost of setting up the new tax downwards from £40-45m to £35-40m.
The agency is worried some people will try to dodge tax, when rates differ between Scotland and the rest of the UK, by falsifying their main residence.
HMRC is seeking access to NHS data to cross-reference information about where people live.
The Scottish government is consulting on this issue until 25 February.
A Scottish government spokesman said: "A consultation is ongoing as to whether HMRC should be allowed administrative data to assist their identification of where people should pay tax.
"No decision has been taken."
In his letter, Mr Troup said: "We need to be sure that we have thoroughly explored all the available options for improving the accuracy of information we hold."
HMRC has also floated the idea of using the electoral register.
In a statement to the BBC, the agency said: "The Scottish electoral register could provide a potential list of people who live in Scotland but do not have a Scottish address on our system."
Voter registration is particularly high at the moment following the independence referendum.
But when local authorities suggested using this data to help chase poll tax arrears, the Scottish government strongly objected.