The Glasgow MSP says he wants to return to his former role as a public law Professor at Glasgow University, as well as being able to spend more time with his family.
He insisted that the decision was “personal”, although the loss of such a senior figure will come as a blow to the party.
Tomkins had served as the party’s Constitution spokesman at Holyrood, but was moved to a strategy role by new leader Jackson Carlaw earlier this year.
“Serving as one of our party’s elected representatives for Glasgow has been an enormous privilege and leaving the Scottish Parliament in 2021 will be a wrench,” Tomkins said.
“I am not leaving the Parliament for political reasons. I wish Jackson and his team every success.
“My reasons are personal, to do with the work I want to pursue in the coming years and to do with the kind of father I aspire to be to my four children.
“I know that I will be stepping down in circumstances where the party is so much stronger than it was five years ago.”
He has been a Conservative MSP for Glasgow since 2016 and will return full-time to his role as the John Millar Professor of Public Law at the University of Glasgow.
But he added: “We have a momentous election in only nine months’ time. Between now and then we must all work tirelessly to ensure that we return and elect the highest possible number of Conservative candidates, not only in Glasgow, but right across the country.”
Mr Carlaw, hailed the “immense contribution” which Tomkins has made to the party.
“Adam has been an invaluable source of energy and intellect, compassion and humour,” the Tory leader said.
“While Adam has made important interventions on the union, his work on social deprivation, drug addiction and live music in Glasgow truly demonstrate the breadth of his interests and capability.
“We will all miss him and we wish him well.”
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