Swinton Insurance expects to cut up to 900 jobs and has put 84 branches and a Norwich call centre under review.
Swinton, which employs 3,000 people across the UK, said where possible, affected staff will be deployed into other parts of the business.
The insurance broker, which has more branches than any of its rivals, blamed business switching to the internet.
It said 90% of its customers now buy insurance over the phone or through digital channels.
The 900 roles will be cut by the end of the year.
Gilles Normand, chief executive of Swinton, said: "We are reshaping our distribution model to ensure that we continue to meet the changing needs of our customers in an effective and efficient way.
"This change is difficult for all colleagues, especially those who are directly impacted by the proposals, but is vital if we are to remain competitive in a challenging insurance market."