(CNN) — The latest round of monthly child tax credit payments were distributed to tens of millions of families on Monday. Parents may receive only one more installment next month unless Congress acts.

More than $15 billion was sent to families of roughly 61 million children, the Treasury Department and Internal Revenue Service said. Eligible households have received a total of roughly $77 billion since the first monthly payment in July.

Most parents automatically get the enhanced credit of up to $300 for each child up to age 6 and $250 for each one ages 6 through 17. The IRS is scheduled to send the final payment in mid-December.

The agency also recently added a feature to its child tax credit portal that allows parents to update their estimated income for this year. This is important because the monthly installments have been based on 2020 or 2019 earnings, and those making more in 2021 may be receiving larger payments than they are entitled to. As a result, they could get smaller refunds or even owe taxes when they complete their 2021 returns.

Also, low-income parents who have not submitted tax returns in the past two years and who didn’t use the IRS tool to claim their coronavirus stimulus checks have until 11:59 p.m. ET on Monday to access the non-filer portal to sign up for the payments. Families who do so before the deadline will receive the money in December as a lump sum, the Treasury Department said.

Those who miss the deadline can still claim the credit of up to $3,600 per child if they file a 2021 tax return next year.

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The enhanced child tax credit, which was created as part of the $1.9 trillion coronavirus relief package in March, is in effect only for 2021. It is key to the Biden’s administration’s effort to reduce child poverty.

It could be extended through 2022 under the sweeping $1.9 trillion budget reconciliation package that congressional Democrats are currently negotiating.

“The child tax credit is giving families across America the flexibility to pay for household essentials, school supplies, and other child care needs,” Treasury Secretary Janet Yellen said in a statement. “This tax relief is making a meaningful difference in the lives of millions of children, and it’s crucial for Congress to extend it by passing President Biden’s Build Back Better agenda so families can continue to benefit.”

However, only heads of household earning up to $112,500 and joint filers making up to $150,000 annually would receive the funds in monthly installments next year, under the latest iteration of the bill. Eligible parents with higher incomes would have to claim the credit on their tax return the following year.

The credit would be made permanently refundable so the lowest income families would continue to receive it.

Here’s what else you need to know about the expanded child tax credit

Who qualifies?

The full enhanced credit is available for heads of households earning up to $112,500 a year and joint filers making up to $150,000, after which it begins to phase out.

For many families, the credit then plateaus at $2,000 per child and starts to phase out for single parents earning more than $200,000 or for married couples with incomes above $400,000.

More low-income parents are eligible for the child tax credit because the March coronavirus relief package made it fully refundable. It had been only partially refundable — leaving more than 26 million children unable to get the full credit because their families’ incomes were too low, according to Treasury Department estimates.

About half of Black and Latino children, as well as kids living in rural communities, received only a partial credit or no credit at all because of their families’ low incomes prior to the enhancement, said the left-leaning Center on Budget and Policy Priorities.

Parents who aren’t citizens can receive the payments for their citizen children as long as they have individual taxpayer identification numbers, or an ITIN, and their children have Social Security numbers.

Families can check their eligibility through this IRS website.

How much will I get?

That depends on your household income and family size.

Eligible families can receive a total of up to $3,600 for each child under age 6 and up to $3,000 for each one ages 6 through 17 for 2021. That’s an increase from the regular child tax credit of up to $2,000 for each child up to age 17.

When will I see the money?

Parents have been receiving half their credit on a monthly basis since July. The final payment is set to be distributed on December 15.

They can claim the other half when they file their 2021 taxes next year.

Parents can check if they are enrolled to receive the advance payments at an IRS portal. They can also use it to provide or update their bank account information, mailing address and estimated income for 2021.

Families who want to receive the payments as a lump sum can opt out of the monthly installments at the IRS portal.

RELATED: Monthly child tax credit payments may mean a big tax surprise in the spring for some parents

Some parents may not want to get the monthly payments, particularly if their incomes increase this year. The payments are credits toward families’ tax liability for 2021 but are based on 2020 or 2019 income and household size. Some who get the advance credits could wind up receiving much smaller refunds — or even owing taxes — next spring when they complete their 2021 returns.

Lawmakers, however, protected lower-income parents from potential overpayments. Heads of households making $50,000 or less and joint filers with incomes of $60,000 or less will not need to repay any excess payments.

Do I have to do anything to get it?

The vast majority of families get the credit automatically because they filed 2019 or 2020 returns claiming the credit.

The IRS also sends the payments to Americans who previously used its non-filer portal to register for the stimulus checks.

But families who haven’t filed tax returns recently or used the non-filer tool must take action. They can use another portal to register to receive the enhanced child tax credit but must do so by 11:59 p.m. ET on November 15 to receive the monthly payment as a lump sum in December. Parents can provide the necessary information about their households and, if they choose, their bank accounts so the agency can directly deposit the funds.

The portal directs parents to GetCTC.org. The site, which launched in September, was developed by the nonprofit Code for America, in collaboration with the White House and the Treasury Department. It is available in English and Spanish.

This story has been updated with additional information.

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