The UK is sending emergency drinking water to one of Japan's worst tsunami-stricken regions in response to an urgent request from the Japanese authorities, the Department for International Development (Dfid) said.
The news came as officials warned today that highly radioactive iodine leaking from the Fukushima Dai-ichi power plant could be seeping into seawater further north than previously thought.
One hundred tonnes of the bottled water is being sent from Hong Kong and will arrive in Japan today before being distributed to people living in the Ibaraki prefecture, a badly hit region south of the stricken power plant.
The urgent request for assistance from the Japanese authorities was received late last week, a Dfid spokeswoman said.
The availability of safe drinking water has been restricted in earthquake hit areas due to damaged infrastructure. Concerns remain over radiation levels in water after the coastal nuclear power plant was damaged during the tsunami on March 11.
International Development Secretary Andrew Mitchell said: "Over two weeks on from this terrible disaster, many people are still in the midst of a crisis.
"The destructive nature of the earthquake and subsequent tsunami has damaged essential infrastructure and left many without water.
"Britain is stepping in to help Japan to cope with this shortage, providing much-needed support at this difficult time."
News of high radiation levels in the seawater has added to Japan's mounting problems which include badly miscalculated radiation figures and lack of storage for dangerously contaminated water.
Workers have today started to pump out the radioactive water from inside the plant which must be removed before work to fix the plant's cooling system can continue, nuclear safety officials said.
The effort comes as a Government-commissioned report has called for Britain to improve its disaster response.
The report written by former Liberal Democrat leader Lord Ashdown, warned that the world is facing "a new kind of future in which mega-disasters are going to be more frequent".
Lord Ashdown criticised the leadership of the United Nations as "very weak and very disappointing" and said that whilst Dfid had a "very good reputation", more could be done to embed disaster relief into its core operations.
On March 12 the UK Government sent a team of 59 rescue specialists, two rescue dogs and four medical staff to assist in Japan's rescue efforts.
The team spent three days searching two towns in Northern Japan, Ofunato and Kaimaishi, where tens of thousands were missing.
They returned to the UK on March 19.