RICHMOND — Virginia’s rainy day fund will top $1 billion by the close of the two-year budget that outgoing Gov. Robert F. McDonnell will propose in December, his office announced Friday.

The fund stood at $295 million in July 2010, midway through the Republican’s first year in office. It has since grown to$440 million and will approach $700 million by the end of the current fiscal year in June.

McDonnell will add more than $300 million to the fund in the two-year budget he will propose before leaving office in January. And while the next governor will be free to disregard most of McDonnell’s spending plan, he will have to abide by that part; the Virginia constitution requires that a certain portion of revenues be set aside for the fund.

Based on revenue forecasts, the constitutionally required deposit will be a total of $314 million in the fiscal 2015-16 budget.

Conservative state budgeting and economic growth helped fuel growth in the fund, McDonnell said.

The term-limited governor made the announcement three days before he delivers his final end-of-the-fiscal year address to the General Assembly’s money committees.

McDonnell previously announced that in June, Virginia concluded fiscal 2013 with a revenue surplus of more than $260 million, marking the first time any governor had posted four consecutive revenue surpluses since the mid-1990s administration of governor George Allen (R) .

On Monday, McDonnell will announce the second component to the overall surplus: the amount of savings achieved in state government operations.

“The work to restore the Rainy Day Fund is yet another bipartisan accomplishment in Richmond,” McDonnell said in a statement. “We’ve budgeted conservatively, saved taxpayer dollars and properly planned for the years ahead.”