Cameron will argue that changing economic policy and borrowing more â¿¿ as suggested by critics â¿¿ would burden future generations and leave Britain unable to afford good public services.
"I know some people think it is being stubborn to stick to a plan. That somehow this is just about making the numbers add up....But nothing could be further from the truth," he will say. "My motives for sticking to the plan are exactly about doing the right thing to help families and business up and down the country."
The prime minister's speech will coincide with an announcement that BT is creating more than 1,000 engineering jobs at its Openreach business, with most of the recruits set to install fiber broadband in homes.