The supplier - owned by the energy giant Centrica - said it had been forced into the "difficult decision" by rising costs and was its first for more than three years.
He said that over that period, overall electricity costs have increased by 16 per cent. Customers should not be lulled into a false sense of security.
"It's time switch supplier and send a message that price rises like these just aren't acceptable".
Why are prices going up?
British Gas is the last of the "big six" energy firms to increase prices. This is now coming to an end.
The "big six" energy firm blamed the hike on the increased costs of supplying electricity and defended the move, claiming it had shielded customers from rises since November 2013, but could no longer offset the cost.
Details of the intended price hike also came as British Gas owner Centrica posted its half-year results which revealed that the company had lost 377,000 United Kingdom customer accounts since the start of the year and earnings from its consumer business had plunged by more than a quarter.
The British Gas CEO was Iain Conn's first major appointment.
The company said it expected to return to profit for the full year and will focus activities on higher value small and medium-sized companies, which are generally more profitable for suppliers than large industrial and commercial customers.
It said it will credit the accounts of 200,000 vulnerable customers with £76 to protect them from the price increase.
He added: 'A British Gas price rise is completely unjustifiable.
This means your prices won't fluctuate - which is often what happens with standard tariffs - often the most expensive around.
Expectations of a price rise were fuelled by a website blunder by British Gas staff yesterday. If we assume that rate of increase was the same 2.9% a year from 2014 to 2017 that would have added no more than £10 to a bill, so we're still £79 short.
'It would unfairly hit millions of ordinary families in the pocket while British Gas makes millions in profit and their executives take home millions in pay packages.
"The cheapest tariffs on the market are £844 for the same usage".
Customers on fixed term deals will not be affected.
Here's what you need to know before you do.
The company said it booked net exceptional charges in the first half of 2017 amounting to GBP268.0 million and GBP176.0 million worth of losses from certain re-measurements, compared to the year before when exceptional items only totalled GBP63.0 million and re-measurements generated a GBP704.0 million net gain.
The message from energy experts is not to rest on your laurels; check if you can switch and save now.