Plunging temperatures and high demand for gas-fired power are the main factors behind the low levels, Centrica said, adding that the need to replenish stocks could lead to rising prices ahead.
The UK is heavily reliant on gas for its home heating and also uses a significant amount for electricity generation.
National Grid data on Friday showed that natural gas accounted for 53% of power in the UK's system, with renewables offering just 16% of the country's needs.
Following the UK's decision to ditch carbon intensive coal from its energy mix, extra strain is heaped on gas during cold snaps because wind generation can often be lower due to high pressure weather systems.
Earlier this week, the UK's electricity grid operator issued a rare notice to power firms that sought higher output to prevent a greater risk of blackouts within the network.
As of 9 January, UK gas storage sites "were 26% lower than last year's inventory at the same time, leaving them around half full," Centrica said.
"This means the UK has less than a week of gas demand in store."
