Vertex has been sold to a consortium of US-based private equity firms for £217.5m.
The business outsourcing subsidiary, owners of portal The Exchange and back-office system 1st, is expected to be sold off by the end of March.
The consortium is being led by Oak Hill Capital Partners and includes GenNx360 and Knox Lawrence International.
The total consideration for the sale is £217.5m comprising cash, debt repayment and the retention by the purchaser of liabilities of Vertex, which UU says is not core to its strategy.
United Utilities chief Executive Philip Green says: “We believe this transaction serves the best interests of our shareholders, allowing the group to concentrate on its core skills. We aim to be a world class operator of utility infrastructure.”