Ministers had been warned about sex offenders going missing, the head of the Police Federation has said.
Jan Berry said the federation, which represents rank and file officers, had alerted the Home Office three years ago but it had failed to find a solution.
It follows newspaper reports police forces across the UK have lost track of 322 convicted sex offenders.
Justice minister Harriet Harman denied the government had been complacent but admitted: "We have got to do more".
According to the News of the World, registered sex offenders - including rapists and paedophiles - have used a loophole in the system allowing them to register vague addresses in order to disappear.
Ms Berry told ITV1's The Sunday Edition: "We warned the government about three years ago when they were reviewing the Sex Offenders Register that some of the addresses that people were giving were sometimes a postcode, sometimes a park bench.
"I think they listened but I don't think they had a solution and sometimes if you don't have a solution you don't talk too much about it."
'Not good enough'
Ms Harman said Britain had a good record in monitoring sex offenders.
"Lots of countries who have the same registration procedures struggle with exactly the same thing. Actually, I think we are one of the best internationally.
But she added: "It is not good enough, we have got to do more and we are getting onto it."
Using the Freedom of Information Act, the News of the World asked all 50 forces in England and Wales how many sex offenders are missing.
The Metropolitan Police has confirmed 88 offenders from London were missing.
The newspaper's investigation also revealed West Midlands Police had lost 25 sex offenders and Greater Manchester 18.
Shadow Home Secretary David Davis said he had also warned ministers about problems with the register.
He told the BBC: "It amounts almost to negligence because this issue was raised by me in October of last year, over four months ago.
"Now, frankly, that should have been got a grip of right there and then."
Liberal Democrat leader Sir Menzies Campbell said the revelation was another sign of the crisis at the Home Office.
He told the BBC: "It's yet another thing that will undermine the public's confidence in the Home Office and in the administration of justice".
Michelle Elliott, founder and director of the child protection charity Kidscape, told BBC News she was "shocked and annoyed" at the investigation's findings.
She said the figures for sex offenders who re-offend were "really quite high".
Released sex offenders are supposed to be monitored by officials working under Multi-Agency Public Protection Arrangements (MAPPA).
The News of the World claimed one paedophile who breached register conditions was last year allowed to give his address as "woods" after moving from "a tent near Guildford leisure centre".
According to the newspaper, Cambridgeshire, Cheshire, Durham and Norfolk Police said they had no missing sex offenders in their areas.
Two police forces in Wales - Dyfed Powys and Gwent - did not provide a figure, it reported.
All the police forces refused to give details about the sex offenders or how long they had been missing.