“Build the wall! Build the wall!” was the auditorium-filling chant which soundtracked Donald Trump’s 2016 election campaign.

But the US president’s core electoral promise of a concrete border wall, paid for by Mexico to keep out the “bad hombres”, will be neither paid for by Mexico nor a wall – even if funding for it is eventually approved by Congress.

And Mr Trump has known this since “early on in the administration”, according to outgoing White House chief of staff John Kelly.

“To be honest, it’s not a wall,” Mr Kelly told the Los Angeles Times this weekend, in a wide-ranging interview published a day before his departure.

“The president still says ‘wall’. Oftentimes frankly he’ll say ‘barrier’ or ‘fencing,’ now he’s tended toward ‘steel slats’. But we left a solid concrete wall early on in the administration, when we asked people what they needed and where they needed it,” Mr Kelly said.

When he previously clashed with Mr Kelly over the nature of the wall in January, Mr Trump angrily took to Twitter, writing: “The Wall is the Wall, it has never changed or evolved from the first day I conceived of it.”

But the same month, Mr Trump described a 32-foot high concrete wall during an interview with the Wall Street Journal. “If you have a wall this thick and it’s solid concrete from ground to 32ft high, which is a high wall, much higher than people planned. You go 32ft up and you don’t know who’s over here,” he said.

And as recently this weekend he described the proposed barrier as “the Wall” on Twitter in a rant blaming Democrats for the deaths of children at the border.

Mr Kelly’s comments come as Mr Trump presides over a federal government shutdown now entering its second week after Democrats refused to sanction specific spending on the $5bn (£4bn) wall, although they have indicated they would be open to funding border security in general.

As a result, hundreds of thousands of public workers have downed tools or are working unpaid as the dispute holds up an agreement on the federal government spending bill in the last days of the Republican monopoly on power in Washington. Fifteen government departments and dozens of agencies have been forced to close temporarily.

The president has rejected the Democrats’ offer to keep border security funding at current levels – including $1.3bn (£1bn) for fencing, but not for the wall. He responded to their stance with a stream of furious tweets blaming the party the for deaths of children detained at the border, and by threatening to “close the southern border entirely”.

Speaking about the controversial policy of separating children and parents at the border, Mr Kelly blamed US attorney general Jeff Sessions for the implementation of the policy.

“What happened was Jeff Sessions, he was the one that instituted the zero-tolerance process on the border that resulted in both people being detained and the family separation,” Mr Kelly said. “He surprised us.” However, the former marine general has frequently mentioned the policy as an intentional “deterrent”, even as early as March 2017.

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He also suggested the issue of migration was not solvable only at the border itself.

“If you want to stop illegal immigration,” he said, “stop US demand for drugs, and expand economic opportunity in Central America”.

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