Amber Rudd privately pledged to give the Home Office "teeth" in efforts to remove illegal migrants from the UK, according to a leaked memo.
The Home Secretary set out her vision to "radically reshape" the department's immigration enforcement activities in a letter to Theresa May last January.
Ms Rudd detailed her aim to raise the number of enforced removals from the UK by more than 10%, telling the Prime Minister she would re-allocate £10 million towards the objective.
She wrote: "Illegal and would-be illegal migrants and the public more widely, need to know that our immigration system has 'teeth', and that if people do not comply on their own we will enforce their return, including through arresting and detaining them.
"That is why I will be refocusing immigration enforcement's work to concentrate on enforced removals.
"In particular I will be reallocating £10 million (including from low-level crime and intelligence) with the aim of increasing the number of enforced removals by more than 10% over the next few years: something I believe is ambitious, but deliverable."
The Home Secretary reportedly set out her strong belief in the need to reshape immigration enforcement to focus on "arresting, detaining and forcibly removing illegal migrants", while also emphasising the need to target gangs and criminals profiting from people trafficking and smuggling.
The Guardian reported that the letter was sent on January 30 2017.
Home Office figures show there were 9,966 enforced removals from the UK last year - a fall of 1,005, or 9%, year-on-year.
The leak comes after a week in which Ms Rudd has come under pressure over the Windrush scandal.
But her department insisted the episode should not be conflated with efforts to tackle illegal immigration.
A Home Office spokeswoman said: "Illegal immigration impacts the whole of society, putting pressure on taxpayer-funded public services, leaving vulnerable people at the mercy of exploitative employers or landlords, and at worst, fuelling the abhorrent crimes of modern slavery and human trafficking.
"People from the Windrush generation are of course here legally.
"The Home Secretary has recognised the huge contribution they have made to our society, and has apologised unreservedly to them.
"We are taking urgent action to help them to evidence their legal right to live in the UK, and have set up a dedicated taskforce to do so.
"It is wilfully misleading to conflate the situation experienced by people from the Windrush generation with measures in place to tackle illegal immigration and protect the UK taxpayer.
"It is clearly essential that we continue to take action against people who are here illegally."
It is understood the £10 million referred to in the letter was already part of the immigration enforcement budget and relates to a decision to refocus investigative efforts away from lower-level immigration crime towards the most serious offending and organised criminal groups.
Official statisticians insist it is not possible to accurately quantify the number of overseas nationals in the country illegally.
Last year, a former immigration enforcement chief claimed the figure was likely to run to more than a million.