America's top judicial body agrees to review lawsuit disputing citizenship by birth.

The US Supreme Court has will hear a pivotal case that puts to the test a century-old guarantee: automatic citizenship for those born in the United States.

On his first day in office this winter, President Donald Trump signed an order aiming to end the policy, but the move was struck down by lower courts after constitutional questions were initiated.

The Supreme Court's eventual ruling will ultimately support citizenship rights for the children of foreign nationals who are in the US illegally or on non-immigrant visas, or it will overturn the provision altogether.

Next, the judges will schedule a date to hear oral arguments between the federal government and plaintiffs, which include foreign-born parents and their infants.

A Constitutional Cornerstone

For nearly 160 years, the 14th Amendment has enshrined the principle that anyone born in the country is a citizen, with specific conditions for children born to diplomats and members of invading forces.

"Every individual born or naturalized in the United States, and subject to the jurisdiction thereof, are citizens of the United States."

The contested directive sought to refuse citizenship to the offspring of people who are whether in the US without legal status or are in the country on non-permanent visas.

The United States is among about 30 countries – mostly in the North and South America – that provide instant citizenship to all those born on their soil.