Prices for day-ahead delivery fell by 0.70 pence to 54.90 pence per therm, their lowest level since December 1.
Further along the curve, gas for January delivery was down 0.76 pence at 54.75 pence per therm, while first-quarter gas fell by 0.70 pence to 54.80 pence per therm.
Although demand was above the seasonal norm due to below average temperatures, gas supplies exceeded demand for energy for heating.
The UK gas system was around 9 million cubic metres oversupplied, with flows expected to be 309 mcm/day, and demand at 300 mcm, National Grid data showed.
Flows from Norway via the Langeled pipeline were up on Monday morning after they were disrupted on Friday due to outages.
Flows from the Netherlands through the BBL pipeline were also slightly higher at around 26 mcm/day compared to 22 mcm/day on Friday.
Meanwhile, in the Netherlands, an outage at the Groningen gas field, which has cut output by 10 mcm/day, continues.
Day-ahead TTF gas was down 0.28 euros at 23.15 euros per megawatt hour.
Russia's Gazprom said at the weekend it had received a prepayment for natural gas supplies from Ukraine, paving the way for the first shipments to Kiev since Moscow cut supplies in June.
In Britain's power market, EDF Energy's 550-megawatt Dungeness B-21 nuclear reactor went offline in an unplanned outage on Monday morning. However, day-ahead baseload power was down 0.40 pounds at 45.65 pounds per megawatt hour.
In Europe's carbon market, benchmark EU Allowances (EUA) were down 0.06 euro at 6.60 euros per tonne, their lowest level since Nov. 17, due to weak wider energy prices and little clarity from discussions in an EU committee on the market stability reserve.