UPDATE: Tesco broadband has told the BBC its service is now back online and the problem "was caused by a technical failure at a network level".
Tesco had said that its UK broadband service went down for thousands of customers across the country.
The problem started at 12:00 BST on Tuesday. The firm said up to 10,000 of its 75,000 customers were without service across different areas of the UK.
Tesco is in the process of selling the business to Talk Talk during this year.
An investigation is under way. At 17:30 BST the company suggested in its Twitter page that the "search for the issue is narrowing".
"Some customers are experiencing network problems. This is not related to the migration of customers to TalkTalk," it said in a statement.
"We are liaising with our network partner [Vodafone] to get the issue resolved as quickly as possible and apologise to any customers affected for the inconvenience."
It is the tenth network failure this month according to the company's Twitter page. Customers in London, the south east of England, and in the West Midlands have been affected in the last two weeks.
A Tesco spokesperson told the BBC:
"Tesco broadband uses its Twitter feed to update customers on all network outages as our customers have told us they find this very helpful.
"This means that we report on any network problem even if just a handful of customers are experiencing issues.
"Our fault rate is in line with the industry and we have high customer satisfaction ratings."