Donald Trump’s proposals to deport millions of illegal immigrants from the US could cost up to half-a-trillion dollars, according to an estimate from American Action Forum (AAF), a centre-right public policy institute.

Policing, legal battles, administration, detention and transportation were all factored into the costs and the AAF estimated the process could take around 20 years

Mr Trump has repeatedly expressed his determination to remove undocumented immigrants from the country but with an estimated 11 million living in the US with no leave to remain, the true cost of mass deportation is coming under scrutiny.

Two years ago the AAF estimated it would cost between £100 and £300 billion, but it had subsequently revised the figure.

Once they have been deported, it would take another £350 billion in enforcement costs to prevent them from returning, according to AAF.

These costs do not take into account the potential losses to the US economy through removing millions of undocumented workers, with sectors such as agriculture, construction and hospitality particularly reliant on illegal immigrants.

the AAF estimated the rapid removal of millions of people would shrink the American economy by almost six per cent in 20 years, with 6.4 per cent of workers believed to be undocumented.

This equates to a loss of £1.6 trillion in lost wages and spending.

Mr Trump's Homeland Security Secretary John Kelly said there would be “no mass deportation” after he met with Mexican officials last week.

The President has also indicated he will concentrate on deporting two to 3 million undocumented migrants who he believes have criminal records.

In a speech to the Conservative Political Action Conference in Maryland, Mr Trump said his administration would concentrate on “throwing” out gang members, drug dealers and criminal aliens, and “we will not let them back in”.

He said his administration has a plan for “swift and strong action to secure the southern border” by means of the wall he has pledged to build - which will cost an estimated £20 billion.

But there is widespread confusion and panic as to what Mr Trump’s policy will be regarding undocumented migrants with no other criminal history after memos from the President directed border control to start rounding up all illegal immigrants.

In his speech to the CPAC, Mr Trump said: “Remember, we are getting the bad ones out. These are bad dudes. We’re getting the bad ones out, okay? We’re getting the bad – if you watch these people, it’s like, oh gee, that’s so sad.

“We’re getting bad people out of this country, people that shouldn’t be – whether it’s drugs or murder or other things. We’re getting bad ones out. Those are the ones that go first, and I said it from day one. Basically all I’ve done is keep my promise.”

Although his speech did not mention the costs of identifying and rounding up illegal immigrants, Mr Trump said that stopping the flow of illegal immigration “would save countless tax dollars”.

Any changes in existing law over the deportation of immigrants will require the approval of Congress and cannot simply be done by means of an executive order from the President.

Mr Trump’s memos to Homeland Security did not address how he intends to deal with an estimated 750,00 young immigrants who were brought into the country illegally by their parents, referred to as “dreamers” because of protection sought for them under the so-called DREAM act.

A series of immigration raids across the US this month have sparked fears of a crackdown after around 680 people were arrested – of whom around a quarter were said by Homeland Security to have no criminal convictions.

Marielena Hincapié, executive director of the National Immigration Law Centre criticised Mr Trump's memos.