Customers could be stuck on pricier energy tariffs for up to an extra three weeks as all gas switches are delayed while an IT system is updated.
The industry-wide IT system, that allows companies to share customer data and exchange gas meter information, will be unavailable until 31 May, in order for the system to be replaced.
It will then take another week to catch up on the lost time meaning the gas switching process will not return to normal until 6 June.
The energy markets watchdog Ofgem has said that customers should not notice any disruption in their supply, but until the switch completes customers will remain on their original tariff.
What does this mean for me?
Consumers switching between providers which are signed up to the Energy Switching Guarantee scheme, must usually be moved over to their new energy supplier within 21 days, but this has been extended for the work to take place.
Any households which are currently in the middle of switching their gas, will see the changeover delayed, and some customers moving both gas and electricity to a new provider will also see delays in their electricity supply changing over as well, due to those companies own procedures.
Ofgem has told MoneySavingExpert that those who applied or are planning to apply to switch suppliers between 23 May and 6 June should only notice delays of up to three days on top of the 21-day period currently permitted.
However, those who applied to move before the 23 May could see longer delays, with some of the smaller energy providers such as Engie telling us it could be an extra three weeks and Octopus predicting two week delays, on top of the usual 21 day switch period.
Ofgem said switchover periods will vary from provider to provider during this time, based on the companies' own systems.
You may struggle to cancel a switch if you've already applied, with energy company Engie saying customers who tell their suppliers they want to cancel their switch during the upgrade period will have to wait until after 6 June for the process to start.
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Why is the work taking place?
The upgrade programme, named Project Nexus, will allow companies to exchange more usage information and make it easier for more consumers to use smart meters.
The spokesman said: "This change will have longer term benefits for consumers by making the systems that underpin billing and switching more efficient.
"There should be no discernible industry wide impact on consumers other than they may experience a slight delay to their switching journey during the period the systems change over.
"We continue to encourage consumers to shop around for the best deal for their energy. There are many suppliers to choose from, so consumers could always look for one that is able to process a request to join them more quickly."