The US defence secretary has agreed to free up $3.6 billion (£2.97 billion) from the Pentagon budget for the Mexico border wall by effectively defunding 127 military construction projects.

Mark Esper determined that the use of the military construction funds was necessary to support American forces deployed to the southern border under the national emergency that Donald Trump declared in mid-February.

The formal determination allows the US president, under an obscure statute in the federal code overseeing the military, to reallocate the funds without approval from congress.

Pentagon officials said the $3.6bn will fund 11 projects providing 175 miles of new or reconstructed barriers along the border with Mexico and help reduce the need for American troops.

Barriers will be reinforced or replaced with bigger walls in some places, while stretches of barrier will be built in other locations.

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Among the projects is a new wall along the US military’s Barry M Goldwater Bombing Range in Arizona, which borders Mexico.

“This funding will all go to adding significantly new capabilities to [the Department of Homeland Security’s] ability to prevent illegal entry,” the Pentagon’s chief spokesperson, Jonathan Rath Hoffman, said. “In areas where we go from, say, a vehicle barrier to a 30-foot wall, we will have a significantly new set of capabilities that didn’t exist previously.”

The Pentagon will take the billions from 127 military construction projects that congress has funded in recent years.

About half of the money will come from projects within the United States and its territories; the other half will come from projects the US military was planning in foreign countries.

The Pentagon declined to disclose which projects would be defunded.

Mr Hoffman said the list would be made available this week, when the US Defence Department will notify lawmakers with affected projects in their districts and foreign embassies.

The Pentagon said the projects were not being cancelled and would not be delayed so long as congress agreed to “backfill” the funds.

“The way we are describing it is really ‘deferred’,” Elaine McCusker, the deputy undersecretary of defence, said. “If Congress were to backfill the projects in our request, none of the projects would be delayed. But we do realise that this could cause some delay. They’re definitely not cancelled.”

Democrats have balked at the suggestion, noting that congress funded the projects in question and should not have to fund them again.

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“The administration’s irresponsible decision to transfer funds from appropriated US military construction makes America less safe and dishonours the constitution,” house speaker Nancy Pelosi said.

She said cancelling military construction projects ”will undermine our national security and the quality of life and morale of our troops”.

Senator Jack Reed, the ranking Democrat on the Senate Armed Services Committee, warned that future presidents would try to tap money for their own desired initiatives through emergency declarations if courts don’t rule against the administration’s border emergency.

“With this move, President Trump is short-changing our troops and taxpayers and forcing them to bear the burden of his broken, preposterous campaign promise,” Mr Reed said.

“Defence spending is supposed to be for national defence. There is no credible reason to divert these funds, and doing so in this manner could disrupt national security efforts.”

Military construction money finances projects range from renovating schools on US military bases to expanding naval piers to accommodate more submarines.

An analysis by The Washington Post this year found that projects in Puerto Rico and initiatives to help European nations deter Russia were particularly vulnerable to being defunded.

The Pentagon said the projects were not being scrapped and that money would be spent only as it was needed to fund the wall contracts. It also suggested it would be seeking ways for congress and foreign nations to still pay for affected projects.

“These projects are important,” Mr Hoffman said. “The intent in prioritising funds in this manner is to provide time to work with congress to determine opportunities to restore funds, as well as work with our allies and partners on improving cost burden-sharing for the overseas construction projects.”

House Democrats, who have uniformly opposed Mr Trump’s emergency declaration at the border, have vowed not to “backfill” the money.

There are about 3,000 active-duty troops and 2,000 National Guardsmen serving on the US border.

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Mr Trump deployed them to help support US Customs and Border Protection, which has been coping with large numbers of arrivals of primarily Central American migrant families.

A statute in US code, known as Section 2808, allows the defence secretary, during national emergencies requiring the use of the armed forces, to carry out construction projects in support of the troops without the approval of congress.

Mr Trump, who is scrambling to fast-track border barrier construction before next year’s election, announced the national emergency after the longest government shutdown in US history, which stemmed from his dispute with congress over how much to fund the projects.

The US president said he declared the national emergency so he could build the wall faster. “I could do the wall over a longer period of time,” he said. “I didn’t need to do this, but I’d rather do it much faster.”

The projects will mark the first time that the administration has tapped the Defence Department dollars that the national emergency allowed Mr Trump to access since the declaration in February.

The Trump administration has previously used a different law that did not require a national emergency declaration to access Pentagon funds for the barrier.

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The administration has allocated about $2.5 billion (£2.06 billion) from the Pentagon budget for wall-related construction projects in Arizona, California and New Mexico using a counterdrug law, which permits the Defence Department to construct fences in drug-smuggling corridors for federal, local, tribal and state agencies.

The administration is also planning to use $601 million (£495 million) from the Treasury Forfeiture Fund for construction of the barrier.

The American Civil Liberties Union has said it will continue to challenge Trump’s use of emergency authorities to tap Pentagon funds for the wall in court.

“The fact that the government sat on these so-called ‘emergency funds’ for seven months further confirms that this is nothing but an unlawful power grab,” said Dror Ladin, staff attorney with the ACLU’s National Security Project. “We’ll be back in court very soon to block Trump’s latest effort to raid military funds for his xenophobic wall."