“If markets rise, the safe withdrawal rate falls because expected returns decline,” the paper says. “Yet if markets fall, the reverse happens and the safe withdrawal rate rises”.

As such, withdrawal rates, and other factors including portfolio composition, can affect the likelihood that a pensioner will not run out of money over a certain period. For a portfolio with a 20 per cent weighting to equities, for example, a £1m pot with a 3 per cent withdrawal rate has a 99 per cent chance of lasting for 20 years. But over a 40-year period, the rate with the same likelihood of success is just 1.4 per cent.

As Morningstar notes, the 'safe' rate can change due to a number of factors and needs regularly monitoring. This illustrated by the firm's own research on the subject: a Morningstar paper published in May 2016 found that the safe withdrawal rate was higher, but it has since fallen 'as prices outstripped the fundamentals'.

Now that pensioners have greater flexibility on how to use their retirement savings, the need for income-generating assets is unlikely to abate. Advisers will continue to ask how to deliver a satisfactory level of income for clients, but they should also ask how much is too much.