The deficit of Tesco's defined benefit (DB) scheme has more than doubled to £5.9bn in just seven months, overshadowing the supermarket's sales growth.
In its interim results published on 5 October, Tesco's deficit on an IAS 19 basis grew by £3.2bn from February to August this year, an increase of 123%.
The supermarket chain said while its assets were performing well, lower bond yields were producing headwinds.
Experts said the increased deficit has cast a shadow over news that Tesco's like-for-like sales grew by 1% across the group and 0.6% in the UK during the first half of the year.
Hargreaves Lansdown senior analyst Laith Khalaf said: "The green shoots of recovery continue to sprout at Tesco, but the mammoth in the room is the pension deficit which has more than doubled in just six months, thanks largely to loose monetary policy pushing bond yields down to exceptionally low levels."
The scheme's triennial valuation, which is due to be conducted in March next year and reported in 2018, might make the size of the deficit look more pressing, Khalaf added.
A cash contribution of £126m was made to the scheme as part of the long-term funding agreement signed off with the trustee in 2015.
This agreement says £270m should be pumped into the scheme each year by the company.
Total indebtedness in the company is now £18bn, an increase of £2.5bn since February 2016 due to a rise in the pension deficit, as measured by IAS 19.
Lincoln Pensions managing director Richard Farr said the deficit is "another wake- up call" for well-known UK corporations which have many pension obligations on their books.
Tesco closed the DB scheme in November 2015, replacing it with a defined contribution arrangement.
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