Justice Patrick Keane on Friday afternoon ruled Senator Roberts, who believed he had renounced his British citizenship before last year's election, was a citizen of the United Kingdom at the time of his nomination.
“For the following reasons, I find that Senator Roberts was a citizen of the United Kingdom by descent at the time of his nomination," Justice Keane said.
"Senator Roberts could have made effective inquiries of the British High Commission by which he would have been informed of the steps necessary to renounce his foreign citizenship. He could have obtained and completed a form of renunciation declaration, such as Form RN, and returned it with the required fee to the Home Office as he belatedly did."
Born to an Australian mother and Welsh father in India in 1955, the Queensland senator is one of seven parliamentarians to have their future considered by the High Court, because the constitution bans dual citizens from sitting in parliament.
On Thursday, the court heard Roberts had sent two emails trying to renounce his alleged British citizenship to email addresses that did not exist.
The court heard Senator Roberts said he found the email addresses "from his research on the internet".
Two days after nominating for the party, Senator Roberts sent an email titled "Am I still a British citizen" to two invalid email addresses he thought were linked to the British consulate.
