Millions of Orange and T-Mobile customers could be left with snail-like internet speeds on their handset because their network has failed to update its systems.

While this is a blow to iPhone, Android and other smartphone users, who typically buy expensive handsets to use the internet, customers can take action to boost their speed manually, as we explain below.

The vast majority of the duo's 27 million customers have access to either network after the two firms merged in 2009. Users are automatically connected to whichever of the two has the stronger signal at the time.

The problem is that when an Orange customer is connected to the T-Mobile network, or vice versa, fast internet access via what's called '3G' is unavailable, leaving them lumbering on frustratingly slow speeds.

If they are on their primary network they will get top speeds, where it is available.

This is a different issue to the signal strength required to make calls and texts, which should be good for both sets of customers as they will be connected to the stronger of the two networks.

Everything Everywhere, the name of the combined group, says the problem is unlikely to be rectified until the end of the year.

It admits customers have been unable to access fast internet when on the alternative network since the first batch of users were moved to network sharing in February (see the Orange and T-Mobile signal share news story).

Since then, more and more customers have been automatically rolled onto the shared network facility, with the final batch set to be migrated "soon".

How do I know which network I'm on?

Your mobile should display the name of the network you are connected to.

How do I know my speed?

In technical terms, the issue is Orange and T-Mobile customers can only access 2G data services when connected to their alternative network.

They need to access 3G speeds to get the best-possible internet connection via their mobile. This is normally vital to enable fast downloads or to allow users to stream videos (though it can be expensive on 3G).

The information below is how you can tell what internet speed you're getting, though not all handsets display these features:

  • New mobile (eg, smartphone): Your phone should display '3G' if you're getting 3G speed. If not, you may be on 2G or have no signal at all. Some handsets, such as iPhones, display the letter 'E' to signify slower speeds.
  • Older mobile (an old brick): Some older models may not display the internet signal strength at all but these are unlikely to offer fast speeds themselves, regardless of signal strength, due to lesser hardware capabilities.

Signal strength to make calls and texts is usually highlighted by the number of bars displayed next to the network name.

What can I do?

Wi-fi, where you connect to an alternative internet server, such as your home internet router, is generally faster than using 3G and can cost nothing from a mobile.

If you need 3G access, Everything Everywhere says you have two options to avoid the risk of being lumbered with a slow connection:

  • You can manually alter your phone settings to ensure you only get service from your host network. This is usually under the 'carrier' or 'network' setting. When in the setting, select your chosen network from the list. You can then switch back to 'automatic' once you're ready to get access to both signals again.
  • You can opt out of network sharing by calling your provider, although the downside is you may not always get the strongest phone signal as a result. Plus, it can be a hassle to call to switch back to sharing.

An Everything Everywhere spokeswoman says: "We always aim to offer our customers an excellent experience.

"We are continuing to invest in our networks to improve coverage and increase capacity, so our customers can make calls, send texts and MMS messages, and browse the internet wherever they want to."