A former chief of Turkey's powerful armed forces has been named as a suspect and called in for questioning by investigators probing an Internet campaign to discredit Prime Minister Tayyip Erdogan's AK Party, state media said Tuesday.

General Ilker Basbug, who retired in 2010, is the highest-ranking officer to be caught up in a widening investigation into the so-called Ergenekon network, an ultra-nationalist group accused by prosecutors of conspiring to topple the government.

The Ergenekon case is seen as part of a power struggle between AK, which has roots in a banned Islamist party and swept to power in 2002, and the secularist armed forces, which have ousted four governments in 40 years.

The investigation, carried out by state prosecutors, centres on allegations Turkey's military set up websites to spread anti-government propaganda with the intent of destabilising Turkey.

The armed forces have seen their influence decline in recent years. Reforms aimed at strengthening civilian rule and winning Turkey's membership of the European Union have curbed the power of generals.

State-run Anatolian news agency said an Istanbul prosecutor had sent a notice to Basbug, who was chief of general staff from 2008 to 2010, calling him to answer questions as a suspect.

Television news channels said Basbug would appear on Thursday.

The so-called Internet Memorandum case is just one of many strands of investigations into Ergenekon that began five years ago.

Several hundred defendants, including retired senior officers, lawyers, academics and journalists, have been put on trial in cases relating to the investigation.

Opposition parties have accused the AK-led government of using Ergenekon to go after its critics. The government denies the accusations.

Retired General Hilmi Ozkok, also a former chief of the general staff, testified in the Ergenekon case in 2009 but only as a witness, not a suspect.

(Writing by Ibon Villelabeitia; Editing by Andrew Heavens)