Octopus has announced it will buy Shell's household energy business in the UK and Germany.
Octopus Energy said that it will take over providing energy for 1.4 million homes as well as broadband services for 500,000 customers.
Shell's customers are advised to "sit tight for now" as the deal is not expected to be completed until later this year.
It is the latest expansion for Octopus which bought Bulb after its collapse.
That deal - which rival energy providers challenged in the UK High Court over state support - gave Octopus an additional 1.5 million customers.
With Shell, its total customer base will grow to nearly 6.5 million customers.
Shell and Octopus both refused to disclose how much the business is being sold for.
Octopus said: "There will be a smooth transition and no disruption to customer energy supply, all Shell energy and broadband customers will be contacted about the next steps following regulatory approval.
"Customer credit balances are protected, and will automatically get transferred to their new account with Octopus together with their existing direct debits."
Shell said the sale is part of plan announced earlier this year to exit its household energy businesses in the UK, Germany and the Netherlands.
The companies are hoping the sale will be finalised in the final three months of this year, subject to regulatory approval.
Octopus has grown to become the UK's second largest domestic energy provider. However, its attempt to acquire Bulb faced opposition by the likes of British Gas, which is owned by Centrica, as well as Eon and Scottish Power.
They went to the High Court to argue it was unfair of the government to extend interest free loans to Octopus for rescuing Bulb which collapsed in 2021 following a spike in wholesale energy prices. The firms were ultimately unsuccessful in their legal challenge.
What can I do if I can't afford my energy bill?
Check your direct debit: Your monthly payment is based on your estimated energy use for the year. Your supplier can reduce your bill if your actual use is less than the estimation.
Pay what you can: If you can't meet your direct debit or quarterly payments, ask your supplier for an "able to pay plan" based on what you can afford.
Claim what you are entitled to: Check you are claiming all the benefits you can. The independent MoneyHelper website has a useful guide.