The Supreme Court has allowed a new Trump administration rule, preventing most people at the US-Mexico border from claiming asylum, to temporarily take effect.

Current White House policy aims to deny asylum to anyone who passes through a safe country on the way to the US without seeking protection there first.

The vast majority of migrants at the border come from Honduras, El Salvador and Guatemala, meaning in most cases they will be expected to apply for asylum in Mexico.

The policy is currently subject to legal challenges.

If it is upheld, it would reverse decades of immigration policy and prevent anyone other than Mexicans and Canadians from first seeking asylum in the US.

'Dangerous' overcrowding of migrants in US border facilities Show all 6 1 /6 'Dangerous' overcrowding of migrants in US border facilities 'Dangerous' overcrowding of migrants in US border facilities Photos of detained migrants crowded into cells at the US border - with one holding up a sign reading simply "help" - have been released as part of a new report warning of "dangerous" overcrowding. The memo was sent to the Department of Homeland Security by its Inspector General, containing photos taken at border facilities in the Rio Grande Valley over a week in June DHS/OIG 'Dangerous' overcrowding of migrants in US border facilities Eighty-eight adult males held in a cell with a maximum capacity of 41, some signalling prolonged detention to OIG Staff on June 12 DHS/OIG 'Dangerous' overcrowding of migrants in US border facilities Overcrowding of families observed by OIG on June 11, at a border patrol facility in Weslaco, Texas DHS/OIG 'Dangerous' overcrowding of migrants in US border facilities Migrant families overcrowding a border patrol facility on June 11 in McAllen, Texas DHS/OIG 'Dangerous' overcrowding of migrants in US border facilities Fifty-one adult females held in a cell designated for male juveniles with a capacity for 40 at Border Patrol’s Fort Brown Station DHS/OIG 'Dangerous' overcrowding of migrants in US border facilities Migrant families overcrowding a border patrol facility on June 11 in McAllen, Texas DHS/OIG

A lower-court ruling had blocked the policy from being enforced in some states along the southern border until the matter was resolved.

But the order issued by the Supreme Court late on Wednesday reverses that ruling and allows the policy to go into effect while the appeals process continues.

“BIG United States Supreme Court WIN for the Border on Asylum!” Donald Trump said on Twitter, following the announcement.

Mr Trump’s administration has struck an increasingly aggressive tone on immigration at the southern border.

Most of the people at the US-Mexico border are Central Americans seeking to escape poverty and violence in their home countries.

The majority of this group will be ineligible for asylum in the US under the new rule, as will those from Africa, Asia and South America.

“This is just a temporary step, and we’re hopeful we’ll prevail at the end of the day,” said Lee Gelernt, an American Civil Liberties Union (ACLU) lawyer representing immigration advocacy groups in the case.

“The lives of thousands of families are at stake.”

In May, Amnesty International USA’s advocacy director for the Americas condemned the move.

“This latest policy is a disgusting example of the lengths the Trump administration will go to to deny people protection,” Charanya Krishnaswami said.

Numerous courts have already considered the policy.

A judge in San Francisco first blocked the rule from taking effect in a late July hearing.

That ruling was later narrowed by an appeals court, so it only applied to Arizona and California.

The Supreme Court’s order allows the Trump administration to enforce the rule across the US.

It is unclear how quickly the policy will be enforced and how it fits in with other administration efforts to tighten asylum rules.

More than 30,000 people have already been turned back from the border and sent to Mexico, to wait for the resolution of their asylum claims.

Thousands more are waiting at the border to be allowed to submit a claim.

At the moment many asylum seekers at the border pass an initial screening, known as a “credible fear” interview.

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Under the new policy most will fail unless they have sought and been denied asylum in at least one county on the way to the border.

If they fail, they will be placed in fast-track deportation proceedings and flown to their home countries.