Northeast Utilities won final approval for its $5.03 billion purchase of Nstar, allowing it to complete an 18-month-long process to create New England’s largest utility owner, Bloomberg News reported last week.

The Mass. Department of Public Utilities issued an order approving the transaction and a settlement announced Feb. 15 by the merging companies, state Attorney General Martha Coakley and other officials.

The approval is the last regulatory consent needed for Boston-based Nstar and Northeast Utilities, based in Springfield, Mass., and run from Hartford, Conn. It clears the way for the merger, announced in October 2010, to close by April 10.

To win approval, the companies agreed to a four-year distribution-rate freeze and a one-time $21 million credit that will be available to Massachusetts customers next month, the regulator said in a statement.

The transaction required approval from shareholders as well as Massachusetts, Connecticut, the Federal Energy Regulatory Commission, Nuclear Regulatory Commission, Securities & Exchange Commission and Federal Communications Commission. •