Although it has not yet been publicly launched, the National Allegations Database is already receiving tip-offs at the rate of one every six seconds.
The deluge of claims - the equivalent of 100,000 allegations every year - suggests there are far more illegal immigrants in the country than is estimated.
Immigration officers in charge of the system believe the majority of the intelligence passed to them is genuine, leading to 900 separate raids and 700 arrests in only a few months.
The database, which is expected to be launched within a few months, will store intelligence offered by members of the public and employers.
Some 25,600 allegations - around 230 every day - were received and passed on to the database between mid-December and the end of March this year.
'Please deport me, there's no work in Britain'
16 May 2012
817 foreign criminals avoid deportation after release
15 May 2012
European judges to decide Abu Qatada fate
09 May 2012
Airport queues? Blame the wind, says minister
Heathrow: 'long queues and fewer immigration checks'
10 May 2012
Around 16,000 contained enough information to be sent to immigration teams to carry out full investigations.
Those marked as high priority cases include tip-offs relating to terrorism, firearms, weapons, sexual offences, drugs and child-trafficking.
Campaigners welcomed the introduction of the database, which was revealed in a letter to MPs by the chief executive of the UK Border Agency (UKBA).
Sir Andrew Green, chairman of Migrationwatch, told the Daily Mail: "This is an astonishing response from the public to an invitation that the Government has barely mentioned.
"It is a pointer to the massive number of illegal migrants in Britain."
Rob Whiteman, chief executive of the UKBA, said that 900 "allegation based enforcement visits" had taken place so far - 700 of which were illegal working operations.
He added: "As a result of these enforcement visits, approximately 700 individuals have been arrested for a variety of offences, including illegal entry, overstaying and facilitation."
In 2005, the Home Office estimated there could be as many as 570,000 illegal immigrants in Britain.
Migrationwatch said the figure was more likely to be around 870,000 because the estimate did not include migrants' children.
The response to the database suggests the figure could be even higher.