U.K. arms of the biggest accounting firms have expanded aggressively into the legal services area in recent years, changing their legal status to compete with attorneys and capitalize on their global presence to win multinational clients.

The firms intend to exploit their large international networks and technological expertise to forge niche positions in specific parts of the legal market.

“We are not trying to replicate traditional law firms,” Ed Stacey, head of employment law at the U.K. arm of PricewaterhouseCoopers,...