* Carrefour Q2 sales seen down 1.9 percent
* Casino Q2 sales seen down 2.2 percent
* Casino due after mkt close on July 15, Carrefour July 16
PARIS, July 15 (Reuters) - French supermarket groups Carrefour and Casino are expected to post declines of around 2 percent in second-quarter sales on a year ago, underscoring tough trading conditions in their home market.
Carrefour, the world's second-biggest retailer behind U.S. group Wal-Mart, is seen reporting a 1.9 percent fall to 23.28 billion euros ($32.7 billion), according to the average of estimates from nine analysts polled by Reuters.
The data shouldn't come as a surprise after Carrefour, which employs around 490,000 people in more than 15,000 stores in 30 countries, issued a profit warning at an analyst day on June 30.
At that time, the group said first-half sales should be about 46 billion euros. First-quarter sales were 22.7 billion.
Chief Financial Officer Pierre Bouchut said then that like-for-like sales excluding petrol and adjusted for any calendar impact slid 0.8 percent in the second quarter, less than the 1.3 percent decline seen in the first quarter.
Reported like-for-like sales improved slightly in Carrefour's domestic market, but Spain saw a 6.5 percent decline and Italy a 2.6 percent drop, he said, adding that the group experienced a "difficult market environment" in the first half.
Retailers across the world are struggling in an economic downturn. Carrefour has suffered more than most due to its exposure to weak western European markets and to sales of non-food goods, which have been hit harder than groceries.
Wal-Mart posted a 0.6 percent fall in first-quarter sales, while British rival Tesco reported a 9.7 percent increase, helped in part by favourable exchange rates.
"The pre-announcements of Q2 trading and H1 profits at Carrefour's investor day has put much of the bad news on the table," Bernstein Research analysts wrote in a note.
"We now expect little incremental information from its Q2 trading update."
Carrefour is fighting back by cutting prices, putting pressure on rivals such as Casino. But this has hit profit margins, triggering last month's profit warning.
Three analysts surveyed by Reuters predicted Casino would post second-quarter total consolidated net sales from continuing operations of about 6.8 billion euros, down from 6.95 billion a year earlier -- a drop of about 2.2 percent.
All three analysts expect sales in France to total 4.3 billion euros, down from 4.55 billion a year ago.
They see French like-for-like hypermarket sales including petrol down between 10.5 and 10.8 percent, hit in part by the step up in competition from Carrefour.
"Following a poor Q1 performance and increased price investment in this quarter, we do not anticipate French sales having taken another leg down and look for a performance broadly in line with Q1," Nomura analysts wrote in a note.
"With the competitive environment in France becoming increasingly aggressive and subsequent price investment by Casino, concern remains over the likely margin impact."
Casino shares have underperformed the DJ Stoxx European retail index by about 15 percent this year. Carrefour has lagged by 4 percent, supported in part by its announcement last month of a plan to make 4.5 billion euros of savings, mainly by cutting costs and improving purchasing terms, by 2012. ($1=.7118 Euro) (Reporting by James Regan in Paris; additional reporting by Mark Potter in London; editing by Simon Jessop)