Donald Trump has promised to begin deporting “millions” of people living in the US illegally, on the eve of the formal announcement of his 2020 presidential re-election campaign,

Writing on Twitter late on Monday night, the president, who is seeking to portray himself as a strong and confident candidate to an unconvinced electorate said those entering the US would be “removed as fast as they come in” and said US Immigration and Customs Enforcement (ICE) would begin removals next week.

He also praised Mexican efforts to prevent illegal border crossings.

“Next week ICE will begin the process of removing the millions of illegal aliens who have illicitly found their way into the United States,” the president wrote. “They will be removed as fast as they come in. Mexico, using their strong immigration laws, is doing a very good job of stopping people long before they get to our Southern Border. Guatemala is getting ready to sign a Safe-Third Agreement.

"The only ones who won’t do anything are the Democrats in Congress. They must vote to get rid of the loopholes, and fix asylum! If so, Border Crisis will end quickly!”

Mr Trump campaigned heavily on immigration during his 2016 presidential bid, and is expected to use similar tactics to fire up support among his base ahead of next year’s election.

But despite his campaign talk of building a wall which he said Mexico would pay for and of stopping “bad hombres” coming into the US, immigration from countries south of the US border has risen during his presidency.

Mr Trump recently threatened to hit Mexico with tariffs on goods if the country didn’t improve border enforcement.

The move prompted Mexican authorities to dispatch their national guard and step-up coordination and enforcement efforts.

A Mexican official, who briefed reporters in Washington on Monday, appeared to claim the efforts had resulted in an overnight success. He said US Border Patrol arrested 2,600 people per day after the agreement was struck on 7 June, according to Politico.

This would represent a fall of almost a half from the average of 4,300 border arrests every day in May.