From Saturday 1 June, First Utility customers on the energy provider's former best buy iSave v12 tariff will see the introduction of its previously announced 18% price hike on their bills.
The average customer will be hit with an increase of nearly £200 on their annual energy bills, with the average bill rising from £1,054 to £1,250, according to Uswitch.
As a result of the increase, announced in April 2013, affected customers were allowed to switch providers, without being hit with an exit penalty.
First Utility blamed the price hike on "network costs, the increasing price of wholesale energy and the social and environmental obligations mandated by the government."
Ann Robinson, director of consumer policy at Uswitch, now says: "While some customers may be happy to swallow the increase, many more will be looking to protect themselves by jumping ship to a more competitive provider. If they haven't already done so, they should be shopping around now."
Read Rachel Lacey's blog: Beware First Utility bill spike
What are the best deals on the market?
According to Uswitch, the cheapest deal currently available is Flow Energy's Thames Fixed Online September 2014, with customers paying an average £1,135 for dual fuel.
This tariff offers price protection until 31 August 2014, however if you leave before this date you will be hit with an exit penalty of £30 per fuel.
For consumers who want to play it safe and not get caught out by price rises, the best fixed tariff is EDF's blue and price promise, which is fixed till February 2015, and on average costs £1,192.
The cheapest variable deal is SSE's discount energy bonus, which costs on average £1,146 annually.
You can compare energy suppliers at moneywise.co.uk/compare