LONDON, Jan 4 (Reuters) - The pool of UK government bonds with a negative yield jumped in December and as a share of the overall market hit its highest since September, data from electronic bond trading platform Tradeweb showed on Monday. According to Tradeweb, almost 1.25 trillion pounds ($1.71 trillion) of gilts, or just over 48% of a total market worth roughly 2.6 trillion pounds, had negative yields as of the end of last month. This represented the highest share of the market since September and coincided with a sharp fall in gilt yields in December as concerns about the economic impact of new COVID-19 restrictions grew. The market value of negative-yielding euro zone government bonds on the Tradeweb platform stood at around 6.66 trillion euros as of end-December, the firm added. This made up more than 70% of a total sovereign bond market worth more than 9 trillion euros, but was down slightly from record highs in November. The pool of negative-yielding investment-grade euro corporate bonds in December was broadly unchanged at 1.65 trillion euros, Tradeweb said. (Reporting by Dhara Ranasinghe; Editing by Tommy Wilkes)