British Prime Minister Theresa May, due in Tokyo on Wednesday, is "outraged" by North Korea firing a ballistic missile over Japan, her Downing Street office said.

"The prime minister is outraged by North Korea's reckless provocation and she strongly condemns these illegal tests," a spokeswoman said Tuesday.

"From our perspective, we are willing to continue to work with our international partners to keep the pressure on North Korea."

Nuclear-armed North Korea fired a ballistic missile over Japan on Tuesday in a major escalation that triggered global alarm and a furious response from the government in Tokyo.

May is visiting Japan from Wednesday to Friday as Britain seeks to expand its economic links before it leaves the European Union.

Pyongyang's nuclear and ballistic missile programmes were already on the agenda for the discussions with her Japanese counterpart Shinzo Abe, which are likely to also include talks on defence and security cooperation, and trade and investment opportunities.

The Downing Street spokeswoman said officials from Britain and Japan had been in "constant contact" ahead of the visit.

"We are going on the trip and our plans haven't changed at all," she said.

"Over the next three days, the prime minister will have quite a lot of time with Prime Minister Abe to discuss, among other things, North Korea."

The last time a North Korean rocket overflew Japan was in 2009.