DUBAI, Aug 24 (Reuters) - The deadline for bids to run a "piggyback" telecom service on the network of Saudi Arabia's No.3 mobile operator Zain Saudi has been extended again until December, the industry regulator said on its website.

The deadline was originally June 2014, then Aug. 13, 2015, and has now been extended to Dec. 10, the Communications and Information Technology Commission (CITC) said, "following a number of requests from stakeholders". It gave no further details.

Seeking to increase competition, the CITC instructed the kingdom's three mobile operators to each host a mobile virtual network operator or MVNO, which lease capacity from a host operator, usually paying a percentage of revenue as well as fees.

Former monopoly Saudi Telecom (STC) teamed up with Virgin Mobile Middle East and Africa (VMMEA), part-owned by British entrepreneur Richard Branson's Virgin Group, while No.2 operator Mobily partnered with Jawraa Lebara.

These launched services last year.

But in April 2014 the CITC ordered the MVNO licence for Zain Saudi be retendered, reversing a decision to award it to a consortium led by Dubai-based retailer Axiom.

Zain Saudi did not immediately respond to requests for comment.

The introduction of MVNOs is the biggest change to Saudi's telecom sector since Zain Saudi became its third mobile operator in 2008. The kingdom is only the second Gulf country after Oman to allow MVNOs, which are widespread in Europe, North America and Asia.

(Reporting by Matt Smith; Editing by David Holmes) ((matt.smith1@thomsonreuters.com; 00971506354039; Reuters Messaging: matt.smith1.thomsonreuters.com@reuters.net))