Kenya's High Court on Thursday blocked the government's decision to close the Dadaab refugee camp - the world's largest - and force Somali refugees to return home.

Judge John Mativo ruled that the plan to shut down the camp was unconstitutional, violated Kenya's international obligations and amounted to persecution of refugees.

Dadaab is home to some 256,000 people, the vast majority of them Somalis who fled across the border following the outbreak of civil war in 1991.

The government unilaterally decided to close the camp in May last year, saying it was a terrorist training ground for al-Shabaab Islamist militants.

It repeatedly stated its intention to deport all Somali refugees despite a barrage of objections from rights groups and relief organizations.??

Mativo ruled that "the government decision specifically targeting Somali refugees is an act of group persecution, illegal, discriminatory and therefore unconstitutional".

The shutdown was ordered without proper consultation of people affected by the decision, in violation of the constitutional right to fair legal proceedings, he said in his ruling.

"Hence the said decision is null and void," he said.

The collective repatriation of Dadaab refugees to the borders of their country of origin against their will violated the 1951 United Nations Convention on refugees, he added.

The judge's ruling also blocks the government's decision to disband Kenya's Department for Refugee Affairs.

A source in the state prosecutor's office said the government would "very likely" appeal the ruling in the coming days.

The government has presented Dadaab as a security risk, saying Somali Islamists inside the camp planned the al-Shabaab attacks at Nairobi's Westgate shopping mall in 2013 and the Garissa university attack in 2015, though it has not provided evidence.

The sprawling Dadaab complex near the border with Somalia currently houses some 256,000 people compared to 320,000 in mid 2016.

The UN High Commissioner for Refugees says the numbers have dwindled thanks to voluntary repatriations as well as resettlement in the Kakuma camp in northwest Kenya.