China United Network Communications Group Co Ltd's investment in Telefonica SA, the biggest telecom company in Spain, has been halved in terms of market value, according to a report in caijing.com.

China Unicom bought a Telefonica stake through two deals in 2009 and 2011. The company invested a total of 11.3 billion yuan ($1.80 billion) in the Spanish telecom operator, partly hoping to boost ties with big telecom carriers in developed countries.

However, by the end of September, China Unicom only had 5.57 billion yuan under the category of "available for sale financial assets", which largely comprises its equity investment in Telefonica, caijing.com reported.

Telefonica's shares have decreased in recent months due to the negative impact of the eurozone debt crisis and weak market demand, with foreign investors increasingly reluctant to hold large stakes in firms listed in crisis-hit Spain.

In the first half of the year, Telefonica's operating revenue was 5.8 billion euros, down 14 percent over the same period last year, caijing.com said.

China Unicom had a net profit of 5.4 billion yuan in the first three quarters of the year.