DUBAI (Reuters) - Qatar's government expects to run a budget deficit for at least three years as low natural gas and oil prices weigh on its revenues, the Ministry of Development Planning and Statistics said on Saturday.

In a long-term report on the Qatari economy, the ministry forecast a fiscal deficit of 7.8 percent of gross domestic product (GDP) this year, which would be the first deficit in 15 years and bigger than the deficit of 4.8 percent predicted for 2016 in the ministry's last report published in December.

The deficit is expected to total 7.9 percent of GDP next year before shrinking to 4.2 percent in 2018, the ministry said.

Qatar, the world's biggest liquefied natural gas exporter, is one of the richest of the Gulf states but like its neighbors, it has been pushed into austerity measures this year in an effort to stabilize its finances. More austerity will be needed to achieve the ministry's projections, the report said.

"This estimate assumes that the government pares recurrent spending and caps growth of capital spending below previously programed levels; that there are effective cost reductions in the hydrocarbon sector, which support transfers to the budget; and additional non-oil and gas revenues accrue to the budget."

Some of the projected improvement in the fiscal balance depends on a hoped-for rise in energy prices; the ministry assumed the average crude oil price would climb to $48.91 a barrel in 2018 from $45.49 in 2017 and $37.88 this year.

The ministry predicted Qatar's economy would grow 3.9 percent this year, down from a previous 4.3 percent forecast. It expects growth of 3.8 percent next year and 3.2 percent in 2018.

Liquidity in the Qatari banking system has tightened and money market rates have risen because of reduced inflows of gas and oil money. The ministry said the central bank might take several steps to reduce pressure on liquidity.

It could cut official interest rates, continue to suspend domestic Treasury bond issuance while resuming its suspension of Treasury bill issues, or adopt unconventional measures used by central banks in other countries such as direct purchases of commercial bonds and extraordinary loans to, or equity injections in, individual banks, the ministry said without specifying which steps were likely to be chosen.

In early 2014, the central bank announced a new loan-to-deposit requirement for banks of 100 percent by the end of 2017. The deposit side of the ratio includes only customer deposits and not long-term wholesale funds, which have recently been the primary source of funding for banks.

The banks are still negotiating with regulators to change the loan-to-deposit formula to include long-term wholesale funds, and the deadline for compliance may be postponed until the end of 2018 because of the liquidity issues at Qatari banks, the ministry said.

(Reporting by Andrew Torchia; editing by David Clarke)