The number of most serious offenders not considered for deportation on release from jail was at least 150, the new Home Secretary John Reid has said.
John Reid visited the team hunting for the former inmates
Mr Reid was speaking during a visit in Portsmouth to the team tracing 1,023 foreign former prisoners.
The new estimate shows the number who had been jailed for serious crimes was higher than a previous figure of 90.
Mr Reid said the final figure could end up being "several hundred" if armed robbers are included in the category.
He said at least 75 in the most serious category had now been found, detained or were under surveillance.
"The numbers who have committed the most and more serious offences has increased as we have studied the cases," Mr Reid said.
"This is a moving picture as we discover more and more. But we are also detaining and discovering more and more."
The foreign criminal deportation row led to the sacking of Mr Reid's predecessor, Charles Clarke, on Friday.
In his first full day in the job, Mr Reid met police, immigration, probation and prisons workers who form the 60-strong team hunting the former inmates released between 1999 and March this year.
He also disclosed more released criminals than previously thought had been found to have re-offended but was unable to detail specific numbers.
Mr Clarke had previously said five of the 79 most serious offenders had gone on to commit crimes.
There have also been two allegations of rape - one of which is still being investigated by police - and an allegation of serious sexual assault.
A breakdown in communication between the Prison Service and Immigration and Nationality Directorate has been blamed for the releases.
It emerged that nearly 300 had been set free even after ministers were alerted.
"The Home Office is obviously a pretty daunting challenge for any minister. But at the moment I think it is even more daunting," Mr Reid said.
"It's so important that I came here on the first day in the job to learn from people here how this process of detection, detention and deportation is going, and obviously to signal to the public that I regard this as my highest priority."
Anne Owers, the chief inspector of prisons, has reported that foreign nationals have not been a priority for the prison or immigration services.
"What I think we can say from our inspections of prisons is that this group of people has not been a great priority for two services working under great pressure," she told BBC Radio 4's Today programme.
Speaking at his monthly news conference, Prime Minister Tony Blair admitted the issue had caused the Labour party "significant damage" at the local elections.
But he defended the workings of the Home Office.
"This is a problem that has existed for decades," he said.
"What happened is that as a result for the first time of proper information being kept the problem was uncovered... changes were then put in place."