The U.K. estimated its population will balloon by 9.9% in the next 15 years, a trend that will both help the economy and exacerbate a rift within Prime Minister Rishi Sunak’s ruling Conservative Party.

The Office for National Statistics projected the number of people in the U.K. will grow by 6.6 million between the middle of 2021 and 2036, almost all of which will come from immigration. Net migration is likely to average 315,000 a year from the middle of 2028.

While this would result in net migration more than halving from last year’s record, overall population growth is stronger than the ONS estimated a year ago. That’s likely to boost the capacity of the economy to grow over the coming years, which will help the Bank of England in its efforts to head off inflation and could bring in more revenue for the Treasury. Yet it cuts against Sunak’s promise to rein in migration.