The Prime Minister today 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.
He 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 Secretary Margaret Beckett said: "Like everyone else we are horrified.
"We are having to check, of course, but if we take this announcement at face value, it does look a very deliberate and flagrant provocation."
Any response to the move would be for the international community to discuss, but she said it would be taken "very seriously".
"I think we are particularly concerned about the impact in the neighbourhood and in the region," Mrs Beckett said on the BBC Radio 4 Today programme.
She said it was difficult to tell how to handle an issue like this without making matters worse.
"I think the international community will have to take very, very careful stock," she said.
"This is very difficult for the neighbourhood, it's very difficult for the region, and what we don't want is to see something happen that will make things even worse and more dangerous than they already are."
Mrs Beckett denied that North Korea's actions were to do with military action against Saddam Hussein or that Britain's authority had been weakened by its involvement in Iraq.
"I don't think that is the case," she said. "After all, it's quite a considerable time now since that happened and there are much more current United Nations statements, resolutions and so on."
Mrs Beckett added: "It's important to try not to make the situation worse, if that is possible.
"Perhaps North Korea has now made that obsolete, that is something that the international community as a whole will have to discuss."
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."
North Korea, led by eccentric pariah Kim Jong-il, has refused for a year to attend international talks aimed at persuading it to disarm.
The country pulled out of the Nuclear Non-Proliferation Treaty in 2003 after US officials accused it of a secret nuclear programme.
Speculation over a possible nuclear weapons test arose earlier this year after reports from the US and Japan of suspicious activity at a suspected underground site.
The official Korean Central News Agency said today: "The nuclear test is a historic event that brought happiness to the our military and people.
"The nuclear test will contribute to maintaining peace and stability in the Korean peninsula and surrounding region."
North Korea is believed to have enough radioactive material for about six bombs, using plutonium from its main nuclear reactor located north of the capital, Pyongyang.
The country also has an active missile programme, but it is not believed to have an atomic bomb design small and light enough to be mounted on a long-range rocket.
If today's test is confirmed, North Korea would become the ninth country in the world known to have nuclear weapons.
The other countries are the US, Russia, France, China, Britain, India, Pakistan and Israel.Reuse content