The government's controversial plans to reform public sector pensions are "fair", the Prime Minister insisted.
Staff from the JobCentre Plus and tax offices will be among 30,000 Scottish workers taking part in the action, along with hundreds of thousands of teachers, lecturers and civil servants south of the Border.
Mr Cameron said the changes being proposed for millions of public sector workers were a "good deal", which would secure affordable pensions for decades to come.
He spoke out as the scale of disruption caused by tomorrow's 24-hour walkout by members of four unions became clear, with thousands of schools, job centres, tax offices and courts set to be closed or badly disrupted.
Driving tests will be cancelled and customs checks at ports affected, while picket lines will be mounted outside government departments.
Mr Cameron, addressing the annual conference of the Local Government Group in Birmingham, said reform was "essential", warning that the pension system was in danger of "going broke" unless action was taken because people were living much longer.
"We just can't go on as we are," Mr Cameron said.
"That's not because, as some people say, public service pensions are ridiculously generous. In fact, around half of public service pensioners receive less than 6,000 a year."