Donald Trump is preparing to ask Congress for $18bn to build his "great wall" on the Southern border with Mexico. Building the wall was one of Trump's election campaign pledges and is expected to put pressure on Washington to back the plan after spending a working weekend with Republicans at Camp David.

The total price tag for the project is $33bn, which includes technology, new roads and maintenance as well as security and training, reported the Wall Street Journal.

According to proposals sent to senators by US Customs and Border Protection, the $18bn would build more than 700 miles of new or replacement barriers but there are also plans to lengthen fencing along the border from 654 miles to approximately 1,000. In total the US-Mexico border measures around 2,000 miles.

It is the first time a number has been put on the cost of building the barrier, which Trump has suggested could be transparent so border patrol gaurds can see "when bags of drugs" are tossed over.

Mexico has repeatedly said it will not pay for the wall but speaking at Camp David, Trump put himself on another collision course with the US neighbour. "I have a very good relationship with Mexico," he said. "But yes, in some form, Mexico will pay for the wall."

Upon leaving, he tweeted: "Leaving Camp David for the White House. Great meetings with the Cabinet and Military on many very important subjects including Border Security & the desperately needed Wall, the ever increasing Drug and Opioid Problem, Infrastructure, Military, Budget, Trade and DACA."

Since announcing his candidacy for the presidency, the wall has never been far from Trump's thoughts and kept his Twitter followers up to date with developments:

Trumps returns Washington after launching an extraordinary defence of his credentials following the publication of a book documenting his first year in the White House.

In "Fire and Fury", author Michael Wolff makes a number of sensational claims including that the then presidential candidate knew his son Donald Jr met with Russian associates, that the Trump campaign was shocked it won the election and doubts from his inner sanctum that he is mentally stable.

In a characteristically bombast series of tweets, Trump called Wolff a "loser" who was denied the access he claimed to have sought. He continued that rather being "baby like" he was "a very stable genius..."