Hundreds of families whose babies died or were injured while in NHS care are currently unaware their cases could be examined as part of an investigation.
An inquiry into failings at Shrewsbury and Telford Hospital Trust (SaTH) had initially focused on 23 cases, but more than 600 are now being looked at.
It comes as the NHS revealed more than 326 new cases of concern, some from 1998, but families have not been told.
The trust will now contact affected families.
An interim report from the continuing inquiry, leaked to The Independent earlier this week, described how children were also left disabled during their time at the maternity unit, where staff got the names of some dead babies wrong.
The review, which covers as far back as 1979, is being led by maternity expert Donna Ockenden.
A letter from NHS Improvement's National Medical Director Prof Steve Powis, to families in July 2019, says it identified a further 326 from its search, which was begun in September 2018, and would be working with the inquiry for "appropriate cases" to be included.
But four months later, it had not yet contacted those families to inform them their cases had been identified and to ask permission for them to be included in the inquiry.
A spokesperson for NHS Improvement said after more people came forward, it was decided to review records to identify any other cases of potential concern.
"Prof Steve Powis is working with families and in agreement with the independent chair, has today written to the trust asking them to hand over all relevant additional records to the review," the spokesperson added.
Paula Clark, SaTH's interim chief executive, said: "Following discussions with NHS England and NHS Improvement it was agreed that the appropriate approach was to fully inform families about the revised terms of reference of the maternity review and to gain their consent to share their records, before releasing their data.
"NHS England and NHS Improvement have now agreed that we can proceed with informing families. We will now be contacting those families to seek that permission."
Rhiannon Davies, who led calls for the inquiry after her baby Kate died in 2009, said she had asked multiple times for the notes to be handed over to the Ockenden review.
Since the details of the report were released on Monday, she and husband Richard Stanton have accused the trust of having a "toxic culture of lying and cover up".
"They've been sitting on the notes doing nothing with them," she said.
Following the publication of the leaked report, the General Medical Council has also confirmed it has asked the NHS for details of any concerns about individual doctors.
"Where we receive details of any such concerns we will take appropriate action to protect patients and public confidence in doctors," a spokesperson said.
"All doctors have a responsibility to take action if they are aware that patient safety may be put at risk."
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