North Korea's ambassador has been summoned to the Foreign Office over Pyongyang's nuclear tests.
North Korea confirmed it had carried out a third nuclear test after monitoring agencies reported an "unusual seismic event".
Foreign Office minister Hugo Swire warned ambassador Hyon Hak Bong that the state faced "increasing isolation and further action" unless it engages with the international community.
The UN Security Council has unanimously condemned the test, which took place on Tuesday, as a clear threat to international security. China also expressed "firm opposition".
Monitors said it appeared twice as big as the previous test in 2009.
A Foreign Office statement said Mr Swire had summoned the ambassador to "underline the UK's firm opposition to this nuclear test and make clear to North Korea that it can either engage constructively with the international community, or face increasing isolation and further action by the international community".
After the meeting, Mr Swire said: "It is essential, as ever, that the international community is united in its approach to this situation and that we speak with one voice.
"I call on 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."
North Korea's state news agency said the test had been "carried out at a high level in a safe and perfect manner using a miniaturised and lighter nuclear device with greater explosive force than previously".
The move was said to be a response to the "reckless hostility of the United States".
Experts suggested the use of a smaller device could mean North Korea is getting closer to building a warhead for a missile.
In his State of the Union address overnight, US President Barack Obama insisted the regime's stance would only isolate it further, and pledged "firm action" to counter the threat.