The deficit of FTSE 350 companies' defined benefit (DB) pension schemes increased by 28 percentage points last year, according to data from Mercer.
The consultant's analysis showed the pension gap for the UK’s 350 largest listed companies grew from £32bn at the end of 2017 to £41bn at the end of 2018.
Mercer attributed the deficit to a £19bn fall in asset values, from £766bn to £747bn, while liabilities remained broadly flat reducing by just £10bn, from £798bn in 2017 to £788bn at the end of 2018.
A Mercer spokesperson said the movements masked a volatile year, with pension schemes in surplus for five months from May to September and the deficit increasing by £24bn to £41bn in December alone.
Mercer suggested the rise at the end of the year was almost entirely due to increasing liabilities as corporate bond yields fell, partially offset by a fall in market implied inflation.
In July, Mercer reported the pension deficit for FTSE 350 companies had more than halved in the first six months of last year.
Le Roy van Zyl, partner at Mercer, said: "2018 was a turbulent year and it is disappointing to see it finish in deficit after finally reaching a surplus for the first time since Mercer began regularly monitoring the position."
He added: "While the return to deficit is unwelcome, we are still in a markedly better position compared to the very large deficit following the 2016 Brexit vote.
"However, the significant volatility demonstrates the importance of schemes locking in gains when opportunities to take risk off the table arise."
Meanwhile Andrew Ward, partner at Mercer, said 2018 was a record year for premiums paid to insurers for buy-ins and buy-outs, with more than £20bn of DB obligations being insured.
He said: "We forecast nearly one third of a trillion pounds to be paid by UK private sector DB pension schemes over a three-year period, from 2019 to 2021.
"While the direction of travel is clear, it is important schemes consider how prepared they are for any market shock.
"With continued Brexit related uncertainty trustees must ensure the risks they’re running are consistent with their objectives and protects their sponsors’ long term financial security."
rachel.addison@ft.com