(Reuters) - Tiffany & Co (>> Tiffany & Co.) lowered its fiscal-year sales and profit forecasts on Thursday, citing slowing economic growth in many countries and weakness in its home market, and the upscale jeweler's shares fell almost 7 percent.

Rival Signet Jewelers Ltd (>> Signet Jewelers Ltd.) also issued a disappointing forecast and posted first-quarter results that showed slowing growth in its U.S. sales, particularly at its higher-end Jared chain.

Tiffany shares were down 6.7 percent in premarket trading, while Signet was off 11.5 percent. Signet shares fell 4.7 percent in London.

Tiffany reported lower-than expected first-quarter earnings and said it now expects global net sales to be up 7 percent to 8 percent for the year, compared with its prior forecast for a 10 percent gain. It lowered its full-year profit outlook by 25 cents a share, to a range of $3.70 to $3.80.

Chief Executive Officer Mike Kowalski said the weakness in the Americas that started in late 2011 remained a "soft trend," and he pointed to "decelerating rates of economic growth in many countries."

In January, Tiffany reported disappointing holiday sales in the United States and Europe, hurt by layoffs and smaller bonuses on Wall Street and in London's City financial center, as well as the debt crisis in the euro zone.

Sales in the first quarter at Tiffany's famed Manhattan flagship store fell 4 percent.

For Signet, whose U.S. chains include Kay Jewelers and Jared, there were also signs of a slowdown. U.S. same-store sales rose 1.2 percent in the quarter. Signet said the timing of Mother's Day reduced that gain by 4.4 percentage points.

Last year, Signet's first-quarter U.S. same-store sales were up 11.1 percent.

The slowness was particularly marked at Jared, where comparable sales in the first quarter edged up just 0.2 percent, compared with double-digit percentage increases last year.

Signet's largest chain, the moderately priced Kay, fared better, with comparable sales up 2.9 percent, echoing the gains reported on Wednesday by mid-tier rival Zale Corp (>> Zale Corporation).

On a brighter note, Signet's British sales, which account for one-fifth of company revenue, rose, continuing to improve despite a difficult retail environment.

And Signet said same-store sales this month are up by "strong double digits."

Excluding the impact of currency, sales at Tiffany stores open at least a year were flat in the Americas and Europe during the first quarter, ended April 30.

Companywide, Tiffany sales increased 7.6 percent to $819.2 million, while same-store sales rose 4 percent. The company continued to get a lift from Asia, excluding Japan. Same-store sales in the region rose 10 percent.

Tiffany reported net income of $81.5 million, or 64 cents per share, for the quarter, up slightly from $81.1 million, or 63 cents per share, a year earlier.

Per-share earnings were 5 cents below what Wall Street analysts were expecting, according to Thomson Reuters I/B/E/S, as the company was hurt by higher product costs.

Signet said it expects earnings per share of 78 to 84 cents in the current quarter -- below the 90 cents analysts project.

(Reporting by Phil Wahba in New York; Editing by Lisa Von Ahn and John Wallace)

Stocks treated in this article : Signet Jewelers Ltd., Tiffany & Co., Zale Corporation