LONDON — What is now Thomson Reuters came to life a decade ago in a $17 billion merger of the Thomson financial information business and the Reuters news service.

Now, the company has reached another potentially transformative deal: selling a majority stake in its financial and risk division to the Blackstone Group, the private equity giant.

A consortium led by Blackstone announced on Tuesday that it had taken a 55 percent stake in the unit in a deal that values the division at $20 billion, including debt. It is the private equity firm’s biggest acquisition since the global financial crisis.

The consortium includes Blackstone, the Canada Pension Plan Investment Board and GIC, a sovereign wealth fund established by Singapore’s government.