Democrats and campaign groups have vowed to fight Donald Trump’s “unlawful” national emergency declaration as he revealed plans to raid existing government funds to build his Mexico border wall.

The US president hopes to gather $8 billion through various means to construct barriers along America’s southern border, declaring at a White House press conference that “walls work 100 per cent”.

Most controversial is the $3.6 billion Mr Trump wants to redirect from existing military construction projects by using a power only available to him after declaring a national emergency.

He also plans to take $2.5 billion from the Pentagon’s anti-drugs fund and $600 million from money forfeited to the Treasury. A further $1.375 billion comes from legislation agreed by Congress in a compromise spending deal.

“We’re going to confront the national security crisis on our southern border and we’re going to do it one way or the other,” Mr Trump said. He added: “We have an invasion of drugs and criminals coming into our country.”