SNP ministers have been warned the problem-hit census could be “worthless” after admitting a failure to achieve a target response rate despite shifting the deadline twice.

Scottish Government Constitution Secretary, Angus Robertson, told MSPs that against a 94 per cent targeted response rate, only 87.9% of households had filled in the census – which he labelled a “good level” of data.

But the Conservatives claimed that the episode has been a “disgrace”, blaming the fiasco on a divergence from the rest of the UK by SNP ministers.

The minister attempted to shift blame away from the Government and the National Records of Scotland (NRS), insisting that “filling in the census is a personal legal responsibility”.

Mr Robertson added: “In line with previous censuses, anyone who has directly refused to fill in the census has now been written to and given a final opportunity to do so before NRS begin the process of referring them for potential prosecution.

“However, decisions regarding prosecutions remain a matter for the Crown Office and Procurator Fiscal Service.”

Speaking in Holyrood, Mr Robertson said he was “pleased to announce” the final response rate of 87.9% - adding that 10 council areas have achieved return rates of more than 90%. Only 25 councils out of 32 achieved the 85% target for local authorities.

The Scottish Government was forced to extend the deadline past the original May 1 date, which allowed around 200,000 extra households, or 8.7%, to complete the document.

Mr Robertson said that NRS regard “this extension to the collection period to have been a success”.

An original budget of £10m for the extension has now been revised to around £6m - taking the total costs for the project to £148m.

An international steering group of experts, set up to carry out the next stage of the census has “acknowledged that we are in a strong position to move forward”, Mr Robertson claimed.

He added: “I am aware that in recent weeks much has been made of the response rate, particularly in light of pre-census targets.

“I would like to take this opportunity to reassure the people of Scotland that a return rate of 87.9% is a good level of national census returns and puts us in a strong position on which to build.

“Through a combination of census returns, individual administrative data, the census coverage survey, and adjustments using aggregate administrative data, NRS will be able to proceed effectively with the next phase of census - to produce the high-quality outputs required by data users.”

But Scottish Conservative shadow constitution secretary, Donald Cameron, highlighted there was no mention of the original 94% target, adding that “the actual national results are way off that number”.

He said: “The Scottish return rate is almost 10% behind the rate in the rest of the UK. Glasgow, our most densely-populated city is a write-off at 81% - not even near what’s required.

“We have had delay after delay after delay and an ever-increasing bill for taxpayers.

“We were told a few weeks ago that Scotland’s census had solid foundations. Scotland’s census lies in ruins – it is a disgrace.

“The SNP could have run the census in sync with the rest of the UK last year but as always, they had to be different – and the cost of that decision is now clear for all to see.”

Mr Cameron asked the minister if he agreed “it is highly probable the census could now be worthless”, raising the prospect of an “interim census” having to be carried out.

But Mr Robertson refuted the claims.

He said: “Whilst 2.3m households did complete the census, sadly 316,000 households did not.

“Yes there are lessons to be learned - but I totally and utterly repudiate assertions of writing off anything in this census. They are false, they are ill-informed and they are misleading.”