The UK's privacy watchdog has warned that plans to create a 'super ID database' may lead to the creation of a national identity number for every Scot by default.
In what critics claim is a devastating blow to SNP proposals, the head of the Information Commissioner's Office in Scotland has called for "suitable safeguards" to be put in place for the plan to give more than 100 public bodies the right to request access to an NHS database containing private information about individuals.
Under the proposals, access would be granted to the HMRC, which would use the information to help build up an accurate database of Scottish residents ahead of the devolution of powers over income tax to Holyrood.
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Other bodies that would be entitled to request access to records and update the database include the police, airports, Quality Meat Scotland and Scottish Canals.
However, in a response to a consultation, Ken Macdonald, Assistant Commissioner for Scotland and Northern Ireland at the Information Commissioner's office, said the proposals could be in breach of European rules and he called for a Privacy Impact Assessment to be carried out.
He went on to raise concerns that extending the database through the use of postcodes, which could be added without the consent of an individual, would be "a shift away from the current consensual model".
"The ICO has concerns as to whether there is a sufficient public interest justification," Mr Macdonald said.
"We do advocate against the creeping use of such unique identifiers to the extent that they could become the national identity number by default.
"If we are to have a national identity number, this should be the subject of proper debate and be accompanied by suitable safeguards. It should not just happen by default."
Critics of the plan said that the comments from Mr Macdonald were "a devastating blow" to the proposals.
The Liberal Democrats, which strongly oppose the plan, are planning to force a vote over the issue in Holyrood tomorrow.
The party's leader, Willie Rennie, said: "Their damning submission to the Scottish Government's consultation backs our calls for work to be undertaken immediately to assess the full risks of proposals to expand access to 120 public bodies and to create a unique reference number for each person.
"Scottish Liberal Democrats will make clear in our debate this week that such significant proposals warrant full scrutiny by Parliament through primary legislation. It would be nothing short of an insult to generations of civil liberties campaigners if we allowed ourselves to sleepwalk towards a national identity number."
The database which Ministers propose to open up includes people's names, addresses and dates of birth as well as information on services they use from various public bodies.
Organisations which would be able to access records under the database include sportscotland, the Scottish Environment Protection Agency and Cairngorms National Park Authority.
The Open Rights Group and the The Scottish Council for Voluntary Organisations have also strongly opposed the proposal to open up the database, warning against the possibility of personal data falling into the wrong hands, while concern has also been expressed about the cost of the project.
Scottish Government sources insisted that no decision had yet been taken over whether to push ahead with the creation of the database, following the closure of a consultation last week.
Advocates of the move believe changes will improve the quality of data held in the register, aiding academic research. Supporters say it will also help in tracing certain people, such as children who are missing within the education system and foreigners who receive NHS treatment in Scotland and leave the country with outstanding bills.
A Scottish Government spokeswoman said: "The consultation has now closed - we are considering the responses carefully and will respond in due course. Identification of Scottish taxpayers and administering the tax are matters for the UK Government and HMRC.
"The NHS Central Register has existed since the 1950s, and is already used by local authorities and health boards under strictly controlled arrangements, to ensure they are dealing with the right individual and to prevent mistakes being made.
"The Scottish Government is opposed to the introduction of ID cards."