President Donald Trump's effort to curtail birthright citizenship has been declared unconstitutional by a second US appeals court, handing him another defeat on a core piece of his hardline immigration agenda.

The Boston-based 1st US Circuit Court of Appeals upheld injunctions won by Democratic-led states and immigrant rights advocates that have stopped the Republican president's executive order from taking effect nationwide.

Trump's order, issued on his first day back in office on January 20, directs agencies to refuse to recognise the citizenship of US-born children who do not have at least one parent who is an American citizen or lawful permanent resident, also known as a "green card" holder.

US Circuit Judge David Barron said Trump's order violated the US Constitution's 14th Amendme

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