TOKYO, July 19 (Reuters) - Japanese shares ended lower on Monday for a fourth straight session, as sentiment was hit by worries over the spread of the highly contagious Delta variant and that the Tokyo 2020 Olympics could worsen the health crisis.

The Nikkei share average fell 1.25% to 27,652.74, falling below a major support level of its 200-day moving average of 27,672 for the first time since early last year.

"The 200-day average is an important technical level. A fall below that could possibly lead to a fall to around 25,500," said Nobuhiko Kuramochi, a senior strategist at Mizuho Securities.

Semiconductor-related stocks led the decline after U.S. tech shares slumped last week, with the Philadelphia semiconductor shares index hitting a one-month low.

Taiyo Yuden lost 3.63%, while Sumco fell 4.1%. Tokyo Electron was down 2.09%, while Advantest shed 1.4%.

The broader Topix lost 1.3%, with all of its 33 industry sub-indexes except the drug sector in the red, in line with shares slipping globally on rising concerns about a surge in coronavirus cases.

Tokyo Olympics organisers on Sunday reported the first COVID-19 cases among competitors residing in the athletes' village, as its population swells ahead of the start of the pandemic-hit Games next week.

"We could see clusters at Olympics, and then more infections in Tokyo. That could lead to political instabilities in Japan," said Naoya Oshikubo, a senior economist at Sumitomo Mitsui Trust Asset Management.

Eroding public support for Prime Minister Yoshihide Suga ahead of an election later this year also weighed on investor sentiment, with a Kyodo poll showing on Sunday that his cabinet's support rate fell to 35.9%, the lowest since he took power last September. (Reporting by Hideyuki Sano and Junko Fujita; Editing by Rashmi Aich and Subhranshu Sahu)