The Home Office lied to EU member states to remove victims of human trafficking and modern slavery in breach of European law, according to whistleblowers.

Legal experts have said the practice is “unthinkable” and “a disgraceful and illegal manipulation of the system”. The mayor of London, Sadiq Khan, has urged the sources to contact Yvette Cooper, who chairs the home affairs select committee. “These are clearly serious allegations which must be properly investigated,” said Khan’s spokesperson.

Whistleblowers allege that, while operating as the third country unit, the now renamed Dublin cessation unit (DCU) regularly lied to other member states and manipulated the system by sending them “extra time” letters, falsely claiming asylum applicants had launched appeals. These letters remove the deadline – usually six months – after which someone seeking asylum can no longer be removed from the UK and sent to the EU country determined to be responsible for assessing their claim.

The practice, which started in 2013, apparently continued until at least December last year, the sources say.

A second disclosure is the claim that administrative failure at the unit resulted in six people being unlawfully held in immigration detention centres in the first week of August last year, for periods ranging from 24 to 72 hours.

The whistleblowers also claim the unit frequently detains applicants without the required timetable in place to deport them, meaning they are released and then detained again shortly afterwards.

The Home Office has denied all the claims. “‘Extra time’ letters can be sent only under specific circumstances, in line with the Dublin regulation, and we comply with this legislation,” a spokesperson said.

“Under the Dublin regulation we may only detain an individual for up to six weeks for the purposes of removal,” they added. “Prior to detention an assessment is made to ensure there is a reasonable prospect of removal. Detention is regularly reviewed in line with policy to ensure it is still appropriate and release will be arranged if the prospects of removal decrease.”

But whistleblowers have told the Guardian the Home Office detains, releases and then detains people again because of administrative errors or escorts not being available. Such behaviour is, according to a specialist lawyer, “an abuse of power and arguably unlawful”.

A spokesperson for Khan said: “The mayor has been clear that the government’s approach to immigration is simply not fit for purpose. Its series of openly hostile policies have had a toxic impact on our country and caused untold distress and heartache to families and people who have lived in and contributed to the UK for years, many of them Londoners. Its inhumane approach towards immigrants must end.”

The DCU is a little known but crucial department that, under the EU Dublin convention, determines which EU member state is responsible for considering an asylum claim and transferring the asylum seeker to that state.

Once the unit has determined which member state has responsibility for someone seeking asylum and made a referral to the national referral mechanism, it has a short period of time to deport them there. Under the Dublin regulation, if the person seeking asylum is not returned within the specified period of time – usually six months – they can no longer be removed from the UK.

The unit can, however, get more time to consider an application if the applicant launches a last-minute judicial review, is in prison or absconds. In this case, it sends the EU member state an “extra time” letter. That letter gives the unit an indefinite amount of extra time before the applicant’s case has to be considered as a UK issue.

But the sources allege that from the introduction of the Dublin 3 regulation in 2013 until at least December last year, if the unit found it did not have enough time to process a national referral mechanism claim – one involving human trafficking and slavery – it sent an “extra time” letter to the member state concerned, falsely claiming the applicant had launched an appeal.

Greg Ó Ceallaigh, a barrister specialising in immigration law at Garden Court Chambers, said this alleged manipulation of the system would be “an extremely serious matter indeed. Staggeringly dishonest and wholly illegal.”

He added: “It is a systematic attempt to circumvent the strict requirements of EU law that were put in place specifically to protect this hugely vulnerable category of people. It is a disgraceful and illegal manipulation of the system.”

Sonali Naik QC, who specialises in immigration, asylum and nationality law, said: “If the Home Office are routinely breaching the six-month window, it is a blatant disregard to the individual rights of asylum seekers and to the UK’s international obligations under the refugee convention and the Dublin convention.

“It’s very serious to make up evidence and to make it up in this context has an added weight, because of the UK’s national and European obligations. It’s disgraceful and an illegal manipulation if they did this.”

The practice only gradually began to stop, the sources claim, when a delegate from the Dublin unit in Germany visited in September 2018. The German delegate “expressed bemusement” at the practice, it is alleged. “It was after this that the practice slowly stopped,” the source claimed.

The second disclosure concerns the unit allegedly detaining applicants in immigration detention centres without the required timetable in place to deport them. This would happen, the sources claim, because of administrative errors or escorts not being available.

“An applicant can be detained for three weeks but then, because somebody at the unit has forgotten to book flights or escorts weren’t available, they are released – and then, two to three weeks later, they are reporting, and all of a sudden they are detained again with no notice or warning.

“Often, the asylum seekers that this practice affected the most are those who are ‘adult at risk, level 2’ – generally those with suicidal thoughts, self-harm issues or recognised as a victim of torture,” it is claimed.

Naik said: “This is evidence of the Home Office not complying with their obligations under article 8 [of the European convention on human rights] to transfer people as soon as practically possible. This means their earlier period of detention is arguably unlawful and certainly unjustified and unnecessary.”

The sources also claimed that administrative failure often results in people being unlawfully held in immigration detention. Last August, for example, six people were allegedly unlawfully held in a single week.

The detentions are unlawful because of administrative failures including files proposing release being sent into storage without being passed to an officer, or, because of the number of releases that need processing, the national asylum allocation unit being unable handle the request.

The Home Office spokesperson said: “Should a file be erroneously sent to storage, this would not prevent action being taken on a case, such as organising a release from detention.”

In April, the Guardian exposed practices in the department alleged by numerous Home Office sources to have resulted in failed deportations and unlawful detentions of vulnerable people.