Customers subscribing to legacy unlimited 4G data plans from Verizon Wireless may start to see slower speeds in areas where there is high demand.

The US number one confirmed in a blog post that its “Network Optimization” programme will apply to 4G subscribers from October. It has been in place for 3G customers since 2011.

The approach sees services slowed for the top 5 per cent of data users on unlimited data plans when connected to cell sites with high demand.

Customers that will be affected consume more than 4.7GB of data in a single billing period on unlimited plans, and have fulfilled their minimum contract terms, meaning they are paying month by month.

The company has been shifting new subscribers to tiered plans, in place of the unlimited option. It stopped offering new unlimited data subscriptions in 2012.

The slower speeds could affect streaming of high-definition video or online gaming, according to Verizon.

The company says Network Optimization differs to throttling as it uses network intelligence to only reduce data speeds if users are causing congestion for others. Once devices are connected to cell sites with lower demand, speeds will return to normal.

Throttling, on the other hand, reduces data speed all of the time, no matter what location the devices are in.