Labour is to guarantee older voters that it will keep benefits such as free TV licences, bus passes and the winter fuel allowance as well as the so-called triple lock guarantee on pensions following Brexit.

As part of a concerted party effort to boost its support among older voters, the shadow chancellor, John McDonnell, and shadow pensions secretary, Debbie Abrahams, are to visit an older people’s centre in south London on Wednesday to make the announcement.

At the 2015 election, Labour’s support among the over-65s – the demographic group most likely to vote – slipped to about 25%. Guaranteeing the triple lock is seen as a way to boost its standing with older voters, but is likely to prove controversial.

The triple lock, introduced in 2010, sets out that state pensions should increase each year by whichever is the highest figure from the increase in average earnings, the consumer price index or 2.5%. Labour had previously said it would keep the system in place over the next parliament.

The triple lock has been criticised as excessively skewing government spending towards an older generation which has, overall, suffered less from economic woes in recent years.

Last month’s report from the Commons work and pensions committee labelled the triple lock “unfair and unsustainable” and questioned other benefits such as the winter fuel allowance.

The triple lock went against the idea of a welfare state “underpinned by an implicit intergenerational contract”, said Labour MP Frank Field, who chairs the committee.

He said: “Each generation is supported in retirement by their in-work successors. But a combination of factors has sent the balance out of kilter. It is now the working young and their children who face the daunting challenge of getting on in an economy skewed against them.”

McDonnell and Abrahams are due to announce the policy on a visit to Southwark Pensioners Centre in Camberwell.

McDonnell said: “After six wasted years of Tory economic failure that has seen some have to wait longer to receive their pension, and those in work approaching retirement having to work longer with earnings stagnating, Labour won’t let them face further uncertainty or reductions in income.”

The imminent departure from the EU meant there was “too much uncertainty for those on fixed incomes”, he said.

Abrahams said: “When taken with our plans to defend the NHS and end the Tory crisis of social care, it is clear that only a Labour government will guarantee a dignified living standard for older people.”