Donald Trump has said “100 per cent” he will declare a national emergency to build his Mexico border wall if he fails to reach a deal with the Democrats to end the government shutdown.

In his strongest comments yet on the matter, the US president said he would “definitely” take the move if the Democrats continued to refuse to given him $5.7 billion for the wall.

Calling a national emergency could get Mr Trump out of his current bind, opening up new powers that could allow him to push forward construction without Congress’s approval.

It has political appeal, as Mr Trump could reopen the government, 25 per cent of which has been shut down for 20 days, while saving face by insisting his wall was still coming.

However the move would almost certainly be challenged in court and break recent historical precedent, likely triggering accusations of executive overreach from his critics.

Speaking to reporters outside on the White House, Mr Trump expressed hope a deal with the Democrats could be reached to get border wall funding and end the shutdown.

But he added: "I have the absolute right to declare a national emergency, the lawyers have advised me.

"I'm not prepared to do it yet ... [But] I may do it. If this doesn't work, probably I will do it. I would almost say definitely."