BRUSSELS--European Union leaders need to find a compromise on the bloc's seven-year spending plan that includes the U.K., Finland's prime minister said Thursday.

"I do hope that we can find a solution that is suitable for the U.K., Jyrki Katainen said. "We are building a budget for the EU27, so we need the U.K. in."

U.K. Prime Minister David Cameron has threatened to veto the EU's 2014-2020 budget unless spending is frozen in real terms, arguing that the EU can't ask for more money from countries struggling to cut their own budgets.

Cameron also wants to keep Britain's rebate, worth 3.6 billion euros ($4.6 billion) last year.

Write to Alessandro Torello at alessandro.torello@dowjones.com