North Korea has sparked alarm around the world by testing a nuclear bomb.
The reclusive communist state has confirmed it has successfully conducted a third underground nuclear test, defying UN orders to stop building atomic weapons.
Foreign Secretary William Hague "strongly condemned" the move, calling it a "violation of United Nations Security Council resolutions".
Pyongyang said the test had been carried out in a "perfect manner."
It came after an earthquake was detected at the country's nuclear testing site.
UN secretary general Ban Ki-Moon said the test was "a clear and grave violation" of UN security council resolutions.
Responding to the news, Hague said: "North Korea's development of its nuclear and ballistic missile capabilities poses a threat to international and regional security. Its repeated provocations only serve to increase regional tension, and hinder the prospects for lasting peace on the Korean Peninsula."
The Foreign Secretary said UN resolutions committed the security council to taking "significant action" in the event of a further launch or nuclear test by North Korea.
"The UK will begin urgent consultations with security council partners calling for a robust response to this latest development," he said.
"North Korea has a choice to make - it can either engage constructively with the international community, cease developing its nuclear and ballistic missile programmes and return to negotiations, or face increasing isolation and further action by the security council and the international community."
North Korea had successfully detonated a "miniaturised" nuclear device in a "safe manner" at a test site in the north east of the country, state media said.
It said the test was aimed at coping with "outrageous" US hostility that "violently" undermined the North's peaceful, sovereign rights to launch satellites.
The United States Geological Survey said yesterday that it had detected a 4.9 magnitude earthquake in North Korea.
The nuclear test is North Korea's first since leader Kim Jong Un took power in December 2011 following the death of his father, Kim Jong Il, and marks a bold statement for the young leader as he unveils his domestic and foreign policy for a country long estranged from the West.
Experts say regular tests are needed to perfect North Korea's goal of building nuclear warheads small enough to be placed on long-range missiles.
This atomic test - North Korea's third since 2006 - is expected to take the capital Pyongyang closer to possessing nuclear-tipped missiles designed to strike the United States.