Donald Trump will make illegal immigration a cornerstone of his 2020 campaign - Getty Images North America

Donald Trump has vowed to begin removing millions of illegal immigrants from the United States starting next week.

The pledge came as Mr Trump formally announced his bid for re-election to the White House in 2020.

He said US Immigration and Customs Enforcement would "begin the process of removing the millions of illegal aliens who have illicitly found their way into the United States," adding: "They will be removed as fast as they come in."

There are an estimated 12 million people in the United States illegally, mainly from Mexico and Central America.

A White House official said the new crackdown would focus on more than one million of those who have already been issued final deportation orders by judges, but still remain in the US.

The official said: "Countless illegal aliens not only violate our borders but then break the law all over again by skipping their court hearings and absconding from federal proceedings.

"These runaway aliens lodge phony asylum claims only to be no-shows at court and are ordered removed in absentia. Enforcing these final judicial orders is a top priority."

It is unusual to make planned immigration raids public before they happen.

But senior White House figures believe a series of mass arrests, and the threat of them, would also act as a deterrent to would-be illegal immigrants, in countries including El Salvador and Honduras, who may be considering attempts to reach the US border.

The number of illegal immigrants from Central America crossing the US border has risen dramatically during Mr Trump's presidency. Funding for his plan to build a border wall has been blocked by Democrats.

He recently secured the co-operation of Mexico by threatening to implement increasingly drastic tariffs on Mexican goods.

The threat was dropped after Mexico agreed to dispatch its national guard to its border with Guatemala.

Three weeks ago there were 4,200 illegal migrants arriving at the US border daily, and that has now dropped to 2,600 a day, according to a Mexican official.

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Mr Trump said: "Mexico, using their strong immigration laws, is doing a very good job of stopping people long before they get to our southern border."

The president said Guatemala was getting ready to sign an agreement that would force illegal migrants to apply for asylum there, rather than trying to head on through Mexico to the US.

Mr Trump said the "border crisis" would end quickly if Democrats would agree to "fix" the asylum system.

Immigration was a central theme of Mr Trump's presidential campaign in 2016, and is expected to be again in 2020.