The Japanese company had said that it was interested in Shire to strengthen its presence in oncology, gastroenterology and neuroscience, which suggests that Shire's divestment of oncology might make it less attractive.
Shire was at pains to point out that it started exploring the sale of oncology in December and commenced the disposal process in January, during which it identified multiple possible US, European and Japanese buyers.
The drugmaker said its board will consider using proceeds for a stock buyback after Takeda decides whether to go through with its contemplated offer.
Still, given the small contribution of the cancer business to Shire's overall profits, Deutsche Bank analysts said this was unlikely to be a deal breaker.
According to United Kingdom takeover rules, Takeda must now either make an offer for Shire or walk away by April 25.
The agreement comes as Takeda's chief executive Christophe Weber is reported to be in the U.S. canvassing investor support for a potential bid for Shire, and will no doubt complicate attempts to put a fair value on the business with just a few days to go before the Japanese company must propose an offer or press pause for six months as required under European Union takeover laws.
At the end of last week, a report from Reuters citing two sources with direct knowledge of the matter claimed Takeda has even gone so far as sounding out potential lenders to help fund the deal.
"This transaction is a key milestone for Shire, demonstrating the clear value embedded in our portfolio", said Shire chief executive Flemming Ornskov.
Shares in Shire were up 1.7% at 3,665.50 pence on Monday.
Jefferies analysts said the sale "should boost Shire's negotiating position on asking price in the current offer period with Takeda".
Last year, Shire's revenues doubled following the takeover to 3.57 billion USA dollars (£2.52 billion), the bulk of which came from the Baxalta business. The portfolio also includes Calaspargase Pegol (Cal-PEG), which is under FDA review for the treatment of ALL, and early stage immuno-oncology pipeline collaborations.