Brexit may be in the rearview mirror for some investors, but it may also loom large for retirement planners.

The decision by U.K. voters to leave the European Union and the impact that may have on interest rates have raised the cost of funding retirement income by nearly 10 percent, according to BlackRock, 

“One of the risks we don’t think that investors and advisors are aware enough of is what happens to the cost of retirement income over time,” says Chip Castille, BlackRock’s chief retirement strategist. “If retirement income is becoming dramatically more expensive—and we’ve seen dramatic price movement since the Brexit vote—it should be something advisors are able to understand and monitor.”

BlackRock helps advisors track the cost of retirement income through it’s CoRI Retirement Indexes, age-based indexes that provide a daily level of how much annual lifetime income a person’s retirement savings can generate.

If a retiree expects to leave the workforce in 2020, the current CoRI Retirement Index 2020 value of $21.49 implies that for every dollar of annual retirement income a person wishes to receive, they would have to save $21.49.

“People are saving for a goal. That goal is usually retirement income,” Castille says. “Retirement income is a moving target that’s been moving a lot since the Brexit vote.”

As voters cast their Brexit ballots on June 23, the BlackRock CoRI Retirement Index 2025 registered at $16.63—meaning that for every $16.63 saved for retirement, someone expecting to leave the workforce in the year 2025 could expect $1 of annual income.

On Friday afternoon, the index had increased to $18.34, meaning that a 2025 retiree would need to save an additional $1.71 for every dollar of retirement income they need, a 10.3 percent increase.

Retirees in the near-term face similar pressures—the CoRI Retirement Index 2018 increased from $21.36 on June 23 to $22.63 on Friday, an increase of $1.27 or 6 percent. Indexes for retirement years between 2007 and 2015 also saw price increases.

“For people already in retirement, there are some things that they have at their disposal to mitigate the CoRI increases,” Castille says. “There may be some part-time income. They can spend less for the near term. There’s also the idea of protecting themselves by considering some kind of insurance-backed product.”

CoRI takes into account interest rates on fixed income investments, inflation and actuarial projections of the two measures over time. Longevity is also taken into account by CoRI, says Castille.

The recent dramatic increase in retirement costs are driven by interest rates, says Castille.

“As rates go down, as they have been over the past few years, the price of CoRI goes up,” Castille says. “Lower rates mean that an investor’s money doesn’t work as well, and the CoRI price movement captures that. When the U.S. 10 Year Treasury went from $1.80 to $1.40 after the Brexit vote, that was a huge drop, and so the CoRI prices went up.”

While CoRI does not consider equity valuations, retirees also have to cope with the rising cost of income-generating stocks and alternatives as interest rates sag. Exacerbating the retirement income problem, returns on the S&P 500 have tended to decline as the CoRI indexes rise.

“The long-term trend in rates has been downward, so the long-term trend in CoRI has been moving up,” Castille says. “In the three years since we introduced CoRI, the price of retirement income has gone up by 40 percent, and the S&P has only gone up about 20 percent. The moderate portfolios that were considered more appropriate for investors in or near retirement are up much less than that. The movement in retirement income cost is being under-reported.

“Imagine a world where you knew the value of your house but didn’t know anything about your mortgage, you just blindly sent checks to the bank and hoped for the best,” Castille says. “We’re in a similar situation here: Everybody knows what’s in their 401(k)s or their IRAs, but nobody knows the size of their liability.”