Customers will save an average £55 a year as a result of the move, according to the company.
Energy companies have been under fire for not passing on wholesale reductions to customers.
Managing director Phil Bentley said: "At British Gas, we know household budgets are stretched, and that our customers are concerned about the effect the recent cold weather will have on their winter fuel bills.
"I’m pleased we're able to offer our customers some extra help with this gas price cut – and that we're able
to do this while it's still winter, allowing our customers to really feel the benefit."
The cut is an average across all regions and all paying methods, although those using pre-payment meters will have a slightly larger
reduction as the firm removes the price difference between pay methods.
British Gas said the bill reduction means it is now on average the cheapest supplier of standard rate gas and electricity.
It is the first of the big suppliers to make an energy price cut this year, and the move could spark cuts among its rivals.