Jeremy Hunt’s promises in his race to be the next Conservative Party leader would cost between £37-65bn, according to the Institute for Fiscal Studies.
Mr Hunt has proposed a corporation tax cut and an increase in the point at which workers pay National Insurance.
He would also raise defence spending and cut the interest on student debt.
Mr Hunt’s campaign said the pledges were “designed to turbocharge the economy attracting inward investment and driving growth.”
However, the IFS concluded the foreign secretary’s plans would leave no scope to relieve the pressure on other spending departments without tax rises or risking higher borrowing.
The IFS has also analysed Mr Hunt’s rival Boris Johnson’s tax plan and said they would cost “many billions” and benefit the wealthy the most.