The Government has been accused of a "masterclass in incompetence" over attempts to improve the criminal records checking scheme.
A project to modernise the Disclosure and Barring Service (DBS) has been marred by poor planning, delays and spiralling costs, according to a Commons report.
The programme is more than four years late and costs are expected to be £229 million more than initially planned, the Public Accounts Committee (PAC) said.
Its assessment also said a new facility introduced to make it easier and faster to check for changes to safeguarding information has seen a "fraction" of the demand expected when it was set up.
PAC chairwoman Meg Hillier said: "Government has a crucial role to play in safeguarding children and vulnerable adults but the handling of this project has been a masterclass in incompetence.
"These are testing times for the Home Office.
"We continue to have serious concerns about its largest project, the Emergency Service Network, which is critical to the ability of our emergency services to do their jobs and keep citizens safe.
"The Department also faces huge challenges arising from the UK's departure from the EU, not least, potential threats to security at the border from day one of Brexit.
"On both DBS and ESN the Home Office appears either to have ignored or not fully understood the needs of the end user."
Established in 2012, the DBS accesses data held on police databases to help organisations make recruitment decisions.
It is widely used in the public, private and voluntary sector, such as schools and care homes, to check prospective staff and volunteers.
Four million disclosures were issued in 2016-17, of which 260,000, or 6.1%, contained information that was "potentially relevant to safeguarding", according to figures published by Whitehall's spending watchdog earlier this year.
A Home Office spokeswoman said: "We recognise that there have been delays in some aspects of the delivery and implementation of the Disclosure and Barring Service's modernisation programme.
"However, the DBS has launched the first phase of its new IT system and will continue to work towards providing their customers with a faster and more efficient service.
"The DBS's safeguarding work is of utmost importance in protecting the public and we continue to work closely with them throughout this period of transformation.
"We will fully consider the Public Accounts Committee's recommendations and respond formally in due course."
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