Canon Inc. was fined 28 million euros ($32 million) by the European Commission for moving forward with a takeover of Toshiba Corp.’s medical systems unit before seeking merger approval.

The Japanese imaging company deployed a tactic known as “warehousing” that was aimed at circumventing requirements to file for approval, the EU’s antitrust arm said in a statement.

The EU has come down hard on companies that don’t strictly follow its merger rules, fining Facebook Inc. for providing misleading information during...