WASHINGTON (Reuters) - The U.S. budget deficit will balloon over the next few years mainly because of deep tax cuts approved in December by Republicans in the U.S. Congress and President Donald Trump , the nonpartisan Congressional Budget Office said on Monday.

Despite stronger-than-predicted economic growth ahead, the CBO said the deficit will grow to $804 billion in fiscal 2018, which ends on Sept. 30, up from $665 billion in fiscal 2017. The deficit is how much Washingtons spending exceeds its revenues.

CBO forecast 3.3 percent growth in 2018 in gross domestic product, a broad measure of the economy, and 2.4 percent GDP growth in 2019.

In the next few years, deficits will grow substantially before stabilizing in 2023, resulting in a projected cumulative deficit of $11.7 trillion for 2018-2027, the CBO said.

(Reporting by Amanda Becker; Editing by Makini Brice and Kevin Drawbaugh)