Mohammed Akunjee, who represents Begum’s family, wrote a letter to home secretary Sajid Javid accusing him of “human fly-tipping” over his decision to strip Begum of her British citizenship.
The Tory leadership candidate made the controversial decision after heavily-pregnant Begum was found living in a refugee camp in Syria.
She later gave birth to a boy named Jarrah – named after a 7th Century Islamic warlord – who died a few weeks after his birth.
Writing in a letter that was published in The Times, Akunjee claimed it is “the responsibility of a British secretary of state to deal with British problems”.
He said: “Your act represents the most profoundly egregious, capricious and politically driven abuse of power.
“It was a unilateral, unprincipled response to the publishing of [a media interview with Begum in the camp] deployed as an artifice or device to further your own personal political objective of being prime minister.
“Ms Begum was a pawn to your vanity. Her baby died.”
Akunjee also slammed the decision to strip her of British citizenship, which Mr Javid was allowed to do because Begum also holds Bangladeshi citizenship.
“Shamima Begum’s parents never contemplated a life for her in Bangladesh. They did not register her birth with the Bangladeshi High Commission, he wrote.
“They did not take her to Bangladesh on holiday as a child. Indeed she has never visited the country.
“Rather, Shamima was born, raised, groomed and radicalised here in the UK.
“The suggestion that Shamima is to you genuinely a Bangladeshi citizen is unsustainable.
“It is the responsibility of a British Secretary of State to deal with British problems.
“Rather than take responsibility for Shamima Begum and her son, you took a British problem and illegally dumped it on our innocent international neighbours.
“Through sleight of hand, you have sought to burden the Bangladeshis with her in the longer term. ‘Your cynical decision amounts to human fly-tipping.”
Begum, 19, originally ran away with two friends to join the evil terror cult when she was a schoolgirl in Bethnal Green in 2015.
She later begged to be allowed to return to the UK – despite talking about how seeing severed heads didn’t bother her.