REUTERS/Yuya Shino
ARM said its board would recommend the all-cash deal to shareholders.
SoftBank will pay £17 a share for ARM - 43% more than ARM's closing share price on Friday and 41% more than ARM's all-time high closing share price.
The deal, first reported by The Financial Times, is believed to be the largest-ever acquisition of a European technology business. It comes just weeks after the
Founded in 1990, the Cambridge-based company designs microchips for a variety of smartphones including Apple's iPhone. But its chips also have the potential to be used in an increasing range of other devices that are starting to come online, from cars and kettles to TVs and fridges.
Shares in ARM jumped as much as 44% on Monday morning (GMT) following the announcement.
While the multi-billion acquisition deal sounds impressive, many will likely see it as another UK tech champion taken out by overseas competition.
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Softbank said it intends to "preserve" the ARM organisation, including ARM's existing senior management team, brand, partnership-based business model and culture.
It added that ARM will remain headquartered in Cambridge following the deal, saying that it expects ARM's 3,000 strong workforce to at least double in size over the next five years.
Masayoshi Son, Chairman and CEO of SoftBank, said in a statement:
"We have long admired ARM as a world renowned and highly respected technology company that is by some distance the market-leader in its field. ARM will be an excellent strategic fit within the SoftBank group as we invest to capture the very significant opportunities provided by the 'Internet of Things'.
"This investment also marks our strong commitment to the UK and the competitive advantage provided by the deep pool of science and technology talent in Cambridge. As an integral part of the transaction, we intend to at least double the number of employees employed by ARM in the UK over the next five years.
"SoftBank intends to invest in ARM, support its management team, accelerate its strategy and allow it to fully realise its potential beyond what is possible as a publicly listed company. It is also intended that ARM will remain an independent business within SoftBank, and continue to be headquartered in Cambridge, UK.
"This is one of the most important acquisitions we have ever made, and I expect ARM to be a key pillar of SoftBank's growth strategy going forward."
Stuart Chambers, the chairman of ARM, said: "This is a compelling offer for ARM shareholders, which secures the delivery of future value today and in cash. The board of ARM is reassured that ARM will remain a very significant UK business and will continue to play a key role in the development of new technology."
It was unclear whether the new government would endorse the deal or try to block it on Monday morning.
But Philip Hammond, the new chancellor, welcomed news, hailing it as the "largest ever investment from Asia into the UK."
"Just three weeks after the referendum decision, it shows that Britain has lost none of its allure to international investors," he said. "Britain is open for business - and open to foreign investment."
He added: "Softbank's decision confirms that Britain remains one of the most attractive destinations globally for investors to create jobs and wealth. And as ARM's founders will testify, this is the greatest place in the world to start and grow a technology business."
Dan Ridsdale, analyst at Edison Investment Research, said: "An increase in inbound M&A was one of the obvious consequences of Brexit and weakened sterling but few expected it to manifest itself so quickly or at so large a scale.
"ARM has always traded at a significant premium rating because it's competitive position in its core market is so secure and because growth is locked in as the company's IP penetrates new domains - such as Automotive and IOT. The company's dominant position and IP business model also mean that there is no obvious cap on how far margins can expand. However there is a psychological limit on how far the multiple can expand on the public markets. There may be some cost synergies, but In paying this multiple, SoftBank is, primarily pricing-in much more of the growth from these new applications that the markets have ever been prepared to do."