A government watchdog sounded alarms on Wednesday over the Census Bureau’s preparations for the 2020 census. A new report found that the agency’s plan faced significant risks just a month before Americans can begin to respond to the critical decennial survey.

The report from the Government Accountability Office (GAO) highlighted numerous cybersecurity and technical challenges that could compromise the “cost, quality, schedule and security of the count”, especially with the new technology introduced this year. Census data – which must be kept confidential by law – is used to allocate $1.5tn in federal funds, help local governments decide where to build roads and schools, and to draw electoral districts. An inaccurate count would be catastrophic.

“Whether through incompetence, or intentional action, this administration’s failures risk causing grave harm to this year’s census and could jeopardize a complete and accurate count,” said Representative Carolyn Maloney, who chairs the House committee on oversight and reform, during a hearing Wednesday.

The warning comes amid heightened concern that many people in the United States, particularly immigrants and people of color, won’t respond to the 2020 census because they are distrustful of the Trump administration.

And this is the first year the Census Bureau will allow people to use the internet to respond to the survey, which goes out to every household. But in a test, connection issues prevented up to 600,000 people from accessing the system at once. So this month, the bureau decided it would use its backup system, called Primus, to collect responses after it encountered issues with the system, according to the GAO report.

But the report expressed concern with the decision to change to Primus particularly because it wasn’t tested during a 2018 full simulation. “Late design changes, such as the shift from one system to another, can introduce new risks, in part, because the backup system was not used extensively in earlier operational testing,” GAO officials wrote. Also in question is an app that census workers are using to collect data.

During the hearing on Wednesday, Census Bureau director Steven Dillingham insisted the bureau’s systems were secured and that it was prepared. And John Thompson, who led the Census Bureau from 2013 to 2017, said in an email he didn’t think the change would be a problem. Primus, he said, was a “good and tested system that has been used for a number of data collection applications”.