More than a fifth of households have so far refused to complete the census ahead of an original deadline of Sunday, after it was delayed by a year by SNP ministers who blamed Covid.
In the rest of the UK, where the census went ahead last year as planned, the census was a major success with 97 per cent of households responding.
Mr Robertson claimed that a 77.2 per cent response rate in Scotland was in fact “a substantial figure” in light of “everything happening in the world right now”.
He confirmed a report in The Telegraph that the window for completing forms would be extended by a further four weeks, beyond the original deadline of Sunday, as more than 600,000 households are yet to respond.
The unprecedented extension will cost an additional £9.7 million, on top of the £21.6 million cost of delaying the census from last year.
The increased costs mean that the botched census is now expected to cost taxpayers £148.3 million in total.
“I understand that many people may be dealing with other concerns. Recent world events have caused anxiety for many and have remained the focus of the media, quite rightly, in recent weeks,” Mr Robertson said.
“Closer to home, people are still dealing with the impacts of Covid-19 and the cost of living crisis and given these challenges, I appreciate another ask of people is difficult.
“However, I cannot stress enough how important it is for the [Scottish] Government to hear the voices of the remaining 604,000 households who are still to return [forms].”
Mr Robertson insisted the decision to put off the census for 12 months had been correct, despite experts believing it was a major factor in the far lower response rate.
A high proportion of census returns – well over 90 per cent -– are usually needed to ensure that data, vital in deciding government funding and policies, is robust.
Mr Roberson said that research would be commissioned to find out why so few people have completed the census despite being sent multiple reminders.
Concerns over ‘nationalist bias’
Guidance informing people they can record themselves as male or female based on their gender identity, rather than biological sex, has also proved controversial.
Others have said they have found it difficult to obtain paper forms, whilst roughly 68,000 people started filling in the census online but abandoned the process without completing it.
Asked what new tactics would be used to drive up response rates over the coming weeks, Mr Robertson urged Conservative MSPs to write newspaper columns encouraging readers to return census forms.
He also offered a “fulsome apology” for this newspaper obtaining details of his statement before they were announced at Holyrood, after he was rebuked by the Presiding Officer.
He promised that an “internal leak inquiry” would be carried out.
‘Disastrous’ handling of census
Donald Cameron, a Scottish Tory MSP, said that the SNP’s handling of the census had been “nothing short of disastrous”.
“It was needlessly delayed by a year and has ended up costing taxpayers an extra £30 million,” he said.
“If the SNP had stuck to the same timetable as the rest of the UK for the census, it would have been completed long before the tragic events in Ukraine unfolded.
“They would have clearly benefited from a UK-wide publicity campaign, which delivered an extremely high response rate elsewhere.
“Despite those warnings, the SNP insisted on going it alone and claimed this would ensure the highest possible response rate. Now we see the exact opposite has occurred.”