"If the UK gives it serious consideration, then the P-1 will garner attention internationally," one Japanese source said.
"It has potential customers beyond the UK, like New Zealand, Norway and Canada, with large maritime areas," said UK consultant Simon Chelton, a former BAE Systems executive and defense attache at the British Embassy in Tokyo.
"We are considering a number of options in regard to defense equipment sales, but we don't comment on specific deals," said Japanese Defense Ministry's spokesman Hirofumi Takeda.
"We are not aware of the discussion so we can't comment," said Teppei Kobayashi, a spokesman for Kawasaki Heavy. "In general it is a matter of national policy, so if there is a decision that results in an order, we will follow that."
Britain's Ministry of Defense said it provided maritime surveillance using a combination of ships, submarines and aircraft, and coordinated closely with allies.
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"We will continue to assess future requirements ahead of a decision in the next Strategic Defense and Security Review in 2015," an MOD spokesperson said.
Senior Japanese and UK officials will have a chance to discuss the idea on Monday at a strategic dialogue in London organized by independent British and Japanese think-tanks.
The P-1, designed to patrol Japan's territorial waters from the Pacific to the East China Sea, where Beijing claims small islands held by Tokyo, will be the country's principal sub hunter for decades to come.
Japan's navy plans to buy around 20, costing about 20 billion yen ($170 million) each, over the next five years, though cracks in the fuselage and wing and engine problems have delayed its entry into service.
No announcement from Britain of any replacement for the Nimrod, which tracked Soviet undersea activity during the Cold War, is expected before May's general election.
Its least risky option could be the Boeing P-8, already built and operated by the United States, the closest ally to both Britain and Japan, the sources said. Deployed by the U.S. Navy last year, the first squadron armed with torpedoes and anti-ship missiles operates from Okinawa in southwestern Japan near China.
Boeing officials in Tokyo were unavailable for comment.
In its most recent order in February, the U.S. Navy said it would buy 16 additional P-8s at a cost of $150 million each.
If Japan can offer a P-1 variant tailored for the British military that is competitive on price and capability, it could represent a viable alternative.
Jointly building a P-1 that taps into Britain's experience building the Nimrod would allow London to retain rights over radar and sensing technology it would lose by buying a U.S. aircraft regulated by the Pentagon, one source said.
Last year Japan and Britain agreed on a deal that will see Mitsubishi Electric partner with European missile maker MBDA to develop a medium-range air-to-air missile for the F-35 stealth fighter, which both countries plan to deploy.