The Prime Minister has condemned North Korea's apparent first nuclear weapons test as a "completely irresponsible act".
Tony Blair also said the test, reportedly held at 2.36am British time, showed the country's "disregard" for the concerns of the international community.
North Korea's Korean Central News Agency (KCNA) said it set off a successful underground blast in defiance of international warnings not to go ahead.
Mr Blair said: "I condemn this completely irresponsible act by the government of the DPRK (Democratic Peoples' Republic of Korea).
"The international community has repeatedly urged them to refrain from both missile testing and nuclear testing.
"This further act of defiance shows North Korea's disregard for the concerns of its neighbours and the wider international community."
Mr Blair added that the test "contravened" North Korea's commitments under the Nuclear Non-Proliferation Treaty and UN Security Council Resolution 1695.
The Foreign and Commonwealth Office (FCO) said the international community would react "robustly" to the test, claimed by North Korea to have been held at an underground site in the north-east of the country.
An FCO spokesman said: "This nuclear test is viewed by the UK, and will be viewed by the rest of the international community, as a highly provocative act to which we will respond robustly.
"It will raise tensions in an already tense region and have repercussions internationally."
