JOHANNESBURG (Reuters) - Africa's third largest mobile operator Celtel is due to double the number of countries where it has abolished roaming charges to 12, a company official told Reuters on Wednesday.

Celtel is due to launch the expansion of its "One Network" on Thursday to cover 400 million people, covering an area more than twice the size of the European Union, the official said.

Both contract and prepaid subscribers will be able to make calls at local rates and receive calls for free across all 12 countries.

Celtel already had scrapped roaming charges across six countries -- Kenya, Tanzania, Uganda, Gabon, Democratic Republic of Congo and the Republic of Congo.

The firm, owned by Kuwait's Zain Group, is now expanding the service to include Burkina Faso, Chad, Malawi, Niger, Nigeria and Sudan.

The Zain Group, formerly known as MTC, plans to expand the area where roaming charges are abolished to more of its operations in Africa and the Middle East, the official said, giving no details.

Celtel, which has more than 24 million customers in 14 countries, is third in Africa behind South African groups MTN and Vodacom, which is jointly owned by Britain's Vodafone Group and South Africa's Telkom.