Meanwhile, Labour has failed to deport almost 150,000 migrants over outstanding 'human rights' claims
More than 50,000 illegal migrants are believed to have fallen off the Home Office's radar - and are at large on the streets of Britain.
Internal Home Office figures revealed 50,000 illegal migrants, including 1,200 foreign national offenders, were listed as "absconded" on Government systems.
The data, obtained by the Independent Chief Inspector of Borders and Immigration, also includes people who were refused asylum then absconded to avoid being sent home - and Channel migrants who have disappeared after placed on immigration bail.
Labour, which has now been in power for just short of two years, managed to blame the Conservatives for their mismanagement of the asylum system in power.
A Home Office source told The Telegraph: "Management of contact with those with no right to be in the UK was chaotic and data was unreliable.
"Under the previous Government, removals failed to keep pace with arrivals, and the Conservatives stopped asylum decision-making as they pursued their failed Rwanda plan.
Shabana Mahmood has doubled the budget for Immigration Enforcement in an effort to ramp up removals - from £681million to £1.33billion.
Enforcement staff numbers are also set to be boosted from 4,500 to 7,300.
The Home Office defines an absconder as a migrant who has escaped from immigration enforcement detention or breached bail conditions, whose whereabouts remain unknown after mandatory attempts to contact by phone and email.
Previous figures from the Migration Observatory at the University of Oxford raised fears that over 100,000 migrants might be living in Britain illegally after not being deported.
Separate documents have revealed that nearly half of identified illegal migrants had not been deported due to outstanding asylum and human rights claims.
Leaked Immigration Enforcement figures dating to August 2024 revealed that 201,926 of 412,191 migrants were listed as "not removable" because of outstanding immigration applications, including asylum claims, appeals and legal challenges.
The majority of those who could not be removed - 149,365 - had outstanding human rights, modern slavery or asylum claims.
The Home Office told The Times the number had fallen since August 2024 and pointed to figures showing 67,188 people had been removed by the Government, a 41 per cent increase on the previous 21-month period under the Conservatives.
The vast majority of the removals - 50,712 - were voluntary deportations, with another 16,476 forcibly deported.
The Home Secretary is set to introduce measures in the Immigration and Asylum Bill which will reform the asylum appeals system and introduce new restrictions on illegal migrant's abilities to frustrate their removal.
Andy Burnham is believed to have given his approval to the Bill after discussions with Ms Mahmood - who is expected to keep her role as Home Secretary in his potential Cabinet.
But the Makerfield MP is said to be considering altering her reforms to Indefinite Leave to Remain which would have doubled the number of years a migrant already in the UK would have to wait before applying for the status.
A Home Office spokesman said the Bill "will see tens of thousands more raids, arrests and deportations of illegal migrants".
The Bill will restrict how judges can interpret Article 8 of the European Convention on Human Rights (ECHR) to stop illegal migrants using it to avoid deportation by tightening its usage to a "core family unit" - spouses, parents and children.
Claims under the Modern Slavery Act will have to be lodged within a certain time after arriving in the UK, with each individual restricted to one claim.
Home Office-appointed adjudicators will replace judges in the asylum appeals systems and new powers will be introduced to fast-track appeals from late claimants.
The Conservatives have argued the only way to regain control of Britain's borders is to leave the ECHR entirely.
Shadow Home Secretary Chris Philp said: "These shocking figures show our system cannot deal with illegal immigrants who have no right to be here.
"The public rightly expects them all to be deported. Last year, the Government only carried out 9,000 enforced deputations. The solution is to end the ability of the courts to interfere with immigration decisions, which cause paralysis and leads to hundreds of thousands [of] illegal immigrants to stay in the UK.
"We also need to leave the ECHR, stop allowing modern slavery clams to enable immigrants to stay here, end asylum claims for illegal immigrants and only allow a small number of genuine claims for legal migrants. If Shabana Mahmood was serious then she would adopt these plans instead of moaning about the past."