Britain summoned North Korea's ambassador on Wednesday after the pariah state sparked global fury with its third nuclear test, the Foreign Office said.
"I summoned North Korea's ambassador today to stress in the strongest terms the UK's condemnation of yesterday's nuclear test," Hugo Swire, a junior foreign minister, said in a statement.
"This is a clear violation of UN Security Council Resolutions, and the UK supports the resolve of the UN to implement more stringent measures against the regime."
Swire called on the international community to "speak with one voice" in its response to Tuesday's nuclear blast, which US and South Korean monitors said was much more powerful than North Korea's previous tests in 2006 and 2009.
The Security Council, of which Britain is a permanent member, united on Tuesday to condemn the test and accuse North Korea of a "grave violation" of UN resolutions.
Swire urged the North Korean regime to "act in the best interests of its people and to choose the path they really want".
"Amid reports of widespread hardship and human rights abuses, the priority must be the health and welfare of North Korean people," he said.