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 the 2014 review of its WhatsApp takeover. It also fined Altice NV last year for similar “gun-jumping” when it implemented...