Stock index futures were flat on Wednesday ahead of private labor market data seen as a precursor to a key government monthly jobs report later in the week, which investors will keenly await after a three-day slide in stocks.

Economists polled by Reuters are looking for a loss of 250,000 jobs in August compared with a 371,000 loss in July when the ADP jobs report is released at 8.30 a.m. EDT. A separate, earlier report showed planned layoffs by U.S. firms fell 21 percent in August.

BP Plc rose 4 percent to $52.49 in premarket New York trading after the company said it had made a giant oil discovery in an area that has assumed increasing importance for Western oil majors.

Shares on ConocoPhillips , which has an 18 percent working interest in BP's Tiber Prospect, rose 1.2 percent in premarket trade. Exxon Mobil Corp rose 0.6 percent.

We will be looking at the ... ADP number, which tends to be a precursor to the change in the nonfarm payroll number at the end of the week, said Art Hogan, chief market analyst at Jefferies & Co in New York.

British Petroleum has made a discovery in the Gulf of Mexico, and they actually define that as being great, so I think that is going to catch some headlines as we work our way through the day.

S&P 500 futures fell 0.5 points and were below fair value, a formula that evaluates pricing by taking into account interest rates, dividends and time to expiration on the contract. Dow Jones industrial average futures were down 3 points, and Nasdaq 100 futures dropped 2.50 points.

U.S. indexes fell 2 percent on Tuesday on uncertainty over the health of banks. Some investors fear the market rally of 50 percent since March has run ahead of economic realities and are predicting a pullback in prices. September is also a traditionally weak month for stocks.

Indexes mostly fell in Europe and Asia on Wednesday, but China's key stock index <.SSEC> closed 1.2 percent higher, led by the banking sector. In Europe the FTSE Eurofirst 300 <.FTEU3> fell 0.6 percent.

There will be a number of releases throughout the day, culminating with the Federal Open Market Committee meeting minutes at 2 p.m., which are closely followed for an insight into the Federal Reserve's thinking on the economy.

The U.S. Labor Department releases revised Q2 productivity and unit labor costs at 8.30 a.m. Economists in a Reuters survey forecast a rise of 6.4 percent in productivity and a 5.8 percent fall in unit labor costs, unchanged from the previous month.

Following that, the Commerce Department releases July factory orders at 10 a.m. Economists in a Reuters survey expect an increase of 2.2 percent compared with a 0.4 percent rise in June.

(Reporting by Edward Krudy; Editing by Padraic Cassidy)