Labour's “open door” immigration policy knowingly risked allowing dangerous people to settle in Britain unchecked, according to documents seen by The Sunday Times.

The Whitehall correspondence, which was illegally withheld by the Home Office for four years, shows how ministers were told by the country’s most senior immigration official that his staff were to be “encouraged to take risks” when granting visas, work permits and extended residency to hundreds of thousands of new migrants.

The cover-up of this policy of risk-taking was so concerted that Richard Thomas, the then information commissioner, sent a team of investigators into the Home Office to trawl all the relevant papers. Earlier this year he rebuked the department for breaking the law and ordered it to release the material under the freedom of information (FoI) law.

The documents help to explain the huge rise in the flow of migrants into Britain as the Home Office rushed to clear a backlog of 45,000 cases.

Officials agreed to fast-track 337,000 applications with minimal checks. This led to a rapid rise in immigration. In 1999, 170,000 visas were granted; by 2002, this had risen to 300,000.

As officials were being ordered to take risks, several potentially dangerous people entered the UK. In late 2001, more than 20 Taliban, who had fled from Afghanistan after their defeat by American and British forces, were allowed to stay in the UK. The documents cast new light on the row over past immigration policy, highlighted by the recent rise of the British National party.

Last week Alan Johnson became the first Labour home secretary to admit the government had made mistakes in its handling of immigration. He said ministers had ignored problems about failed asylum seekers and foreign national prisoners. They had also failed to grasp public unease about the growing pressure on jobs and public services.

Johnson’s remarks signalled the government’s belated recognition that its immigration policy has alienated its white working-class vote, tempting a significant minority to back the BNP. The documents indicate that, far from being a mistake, there was a deliberate policy — apparently endorsed at the highest level in the Home Office — to promote concerted risk-taking by immigration staff whose job was to decide whether non-European Union migrants applying to work, study or marry in Britain were genuine.

A key figure in the scandal was Sir Bill Jeffrey, who was the director-general of the Immigration and Nationality Directorate, Britain’s most senior immigration official. He is now at the centre of controversy as the senior civil servant in charge of the Ministry of Defence.

The other key figure was Beverley Hughes, then minister of state for citizenship and immigration. She was later forced to resign after it emerged she had misled MPs about whether she had been warned that Romanian and Bulgarian crime gangs might want to exploit the UK’s decision to open its borders to those seeking work from eastern Europe.

In March 2003, shortly after the 2001 entry permits to the Taliban had come to light — to an outcry in the press — Jeffrey spelled out the policy in a note to Hughes.