Ahead of a meeting next week to decide the bloc's long-term spending plans, Cameron is facing opposition from a number of European countries over his call for the €1 trillion EU budget to be frozen in real terms.
Monti, a former EU commissioner, maintained his normal diplomatic tone but made it clear that Italy and Britain remained far apart on the issue, despite being respectively the third and fourth largest net contributors to the budget.
"Our positions have elements which do not perfectly coincide, to use British understatement," he said at a joint news conference in Rome.
"Even if we are supporters of a budget focused on the future, we are less convinced than the United Kingdom about the need for a significant reduction in the size of the European budget," he said.
Under pressure from Eurosceptic members of his Conservative party who want budget cuts, Cameron has threatened to veto the EU's seven-year budget if there is no agreement on a real-term freeze.
"We cannot go on increasing EU spending. The European Union has got to start living within its means," he told reporters.
"That means spending efficiently, spending effectively and in prioritising measures that will boost prosperity for all EU tax payers," he said.
"You are one of the larger net contributors in the European Union," he told Monti. "We are as well, I think it is in both our interests that there is proper financial control."