David Cameron has warned North Korea that it would be "unacceptable" for it to defy calls to abandon rocket tests.
Speaking on a visit to Japan, the prime minister urged the regime to "take a different path" and engage with the international community.
Cameron and his Japanese counterpart Yoshihiko Noda discussed the threat from nuclear-armed North Korea when they held talks on Tuesday.
North Korea is putting the finishing touches to a rocket that it says is designed to put a satellite into orbit.
However, the US believes the launch - due this week - is a cover for testing a ballistic missile and Japan has deployed its anti-missile systems.
"On the issue of North Korea we have said very clearly and the Japanese and others have said that it is unacceptable to launch these satellites and missiles," Cameron told TV station News Zero.
"Now there is a new leadership in North Korea they should be taking this opportunity to change their approach, engaging with the rest of the word and stopping this sort of activity.
"If they go ahead it should be condemned in the strongest possible terms by the UN Security Council.
"We have to send the clearest possible message... Clearly North Korea is quite a bankrupt country; they are unable to feed their own people and interestingly America has been helping.
"We need to explain that they need to take a different path. They need to do less in terms of weapons and the military and more engagement with the rest of the world.
"If they launch this satellite, that is clearly a breach of the UN resolution that has already been ratified."