Commissioner Charles Rettig said the delay in the tax filing deadline, which was announced Wednesday, could postpone the tax credits, as it reduces the time the agency has to launch a portal for the child tax credit program.

“The same people who do our income tax processing…are the people who need to develop that portal, so I don’t have the resources to devote to that portal until filing season ends,” he told a House tax subcommittee. “We now have one month less to do the development.”

Rettig did not say the delay was inevitable, but was instead sounding a warning. He said the IRS would do its best to make the deadline but was uncertain of success.

The shorter timeline could also prevent the IRS from making monthly payments initially, he said.

The portal Rettig described is seen as a key part of the program, as it allows Americans to update the IRS about things like income changes, marital status, and other factors that can impact eligibility.

As part of the recently passed economic stimulus package, families are eligible to receive a credit of $3,600 for each child under age 6 and $3,000 for each under age 18 this year. That’s a significant jump from the current credit of up to $2,000 per child under age 17. The credit will be fully refundable, rather than the partial refund currently offered.

The law, as it stands, calls for half the payments to be sent to families on a monthly or periodic basis from July through December. The remaining half will be claimed on 2021 tax returns.

More must-read finance coverage from Fortune:

• Charts: The U.S. will spend billions in stimulus to tackle child poverty, but that’s still way behind other countries
• The daring architecture of Elon Musk’s compensation plan has the Tesla CEO on track to make history
• Famed economist Jeffrey Sachs rails against Bitcoin: Highly polluting and “almost like counterfeiting”
• Will stimulus checks take Bitcoin even higher? One analyst thinks so
• Stimulus checks will be shielded from some debt, but still not collectors