A "disturbing gap" in the Government's proposed changes to the vetting scheme would give paedophiles "a golden opportunity of targeting innocent victims", the NSPCC has warned.
The reforms, announced earlier this month, were part of Government attempts to end a "13-year assault on hard-won British freedoms" and would see only those working most closely with children or vulnerable adults having to undergo a criminal records check, Deputy Prime Minister Nick Clegg said.
But the NSPCC said a loophole in the proposed legislation would give sex offenders access to children.
Under the proposals, a teacher who has been barred but not prosecuted for inappropriate behaviour could become an unpaid, supervised, voluntary worker in a school without any checks revealing the previous behaviour that led to them being barred, the leading children's charity said.
An NSPCC spokesman said: "This is a disturbing gap in the planned legislation which could put children at serious risk of harm. It must be addressed as soon as possible to deny offenders a golden opportunity of targeting innocent victims."
In a parliamentary briefing for MPs, the NSPCC said: "A person might be barred from regulated activity, but would then be able to move into non-regulated activity with children without sanction and without employers necessarily being alerted to the risk they pose.
"For example, someone barred from teaching because of inappropriate relationships with young people could become a voluntary teaching assistant. Under these proposals an employer could conduct an enhanced disclosure check but this will only show criminality information not barring information.
"This is highly concerning as most people who pose a risk to children are not prosecuted, and thus future employers may not be alerted to the risks they pose."
Shadow home secretary Yvette Cooper said: "Before the election Labour was already implementing changes to simplify the system and remove unnecessary checks on recommendation of Sir Roger Singleton (Chief Adviser on the Safety of Children). But, as the NSPCC has shown, some of these new Government changes go too far.
"We will be calling on the Government in the debate to think again and make sure these loopholes are dealt with and child safety is put first."