More than four in five existing benefit claimants in Gwynedd waiting to move to Universal Credit are yet to be transferred.
Some benefit recipients are being transferred to Universal Credit, but payments to many were stopped completely, rather than moved to the new system.
DWP figures show around 1,920 migration notices were submitted to legacy benefit and tax credit claimants in Gwynedd between July 2022 and March. None of these cases were closed, meaning no-one lost their benefits. But Turn2Us said people could be missed as the DWP aims to transfer almost all legacy claimants and all tax credit recipients to Universal Credit by 2024-25.
In Gwynedd, roughly 82% of people sent a migration notice during this period remained on legacy benefits as of 31 March, meaning they could yet be stripped of their support.
All the households on hold had been contacted in the last three months, the time-frame the DWP gives people to migrate to Universal Credit.
The DWP initially estimated 3% of households would lose their benefits when transferring to Universal Credit. It later adjusted this to 26% of tax credit-only households and 4% of those receiving legacy benefits.
Turn2Us said it is "deeply concerned the process of managed migration has seen thousands of people drop off benefits altogether".
Claire Atchia McMaster from Turn2Us said many claiming legacy benefits have complex needs and may lack digital access to manage support.
She added: "It is vital the process of contacting and supporting these people is not rushed, ensuring they are given adequate time, accessible information and personalised support to move over to Universal Credit.
"Disruption to benefits payments alongside the five-week wait for Universal Credit could cause serious harm to their finances and lives."
Those who did not voluntarily transfer to Universal Credit and receive less than their previous benefit support are provided with financial protection. 166,860 households across Great Britain received transactional protection during the same period, a top-up designed to aid people's migration to a lower income – including around 140 in Gwynedd.
A DWP spokesperson said: "The majority of tax credit claimants have successfully moved to Universal Credit.
"Our statistics show that only 30 people logged complaints while over half a million households had been invited to move to Universal Credit by the end of March.
"There is a range of support available to help people move, including extensions for those who need extra support."