Scottish Power to close Longannet coal-fired plant in March 2016

* Cites carbon tax, transmission costs

* Also drops plan to develop Cockenzie CCGT plant

LONDON, Aug 18 (Reuters) - ScottishPower, one of Britain's "big six" energy suppliers, said on Tuesday it will close its Longannet Power Station at the end of March 2016.

The coal-fired power plant will close after 46 years of power production, the company said in a statement.

The Scottish government said it would establish a taskforce to support the workers, businesses and communities affected by the closure of Longannet, which employs 236 people.

The company this year announced it would close the plant after a bid for a contract for grid-balancing services was rejected by National Grid (LSE: NG.L - news) .

Scotland's energy minister Fergus Ewing said: "Today's decision is ultimately an unfortunate and direct result of the UK's discriminatory transmission charging system that penalises Scottish electricity generators in comparison to those in the south of England. In Longannet's case the extra charges amount to 40 million pounds per year."

The combination of high carbon taxes and high transmission charging makes running a thermal plant in Scotland uneconomic, ScottishPower said.

"We have explored every potential option to keep the station open, and we still maintain that Longannet could continue generation in to the next decade under the right economic conditions," Neil Clitheroe, chief executive of retail and generation at ScottishPower, said in a statement.

ScottishPower also said it would not continue with the development of a combined-cycle gas turbine plant at Cockenzie because of the same uneconomic conditions.

(Reporting by Nina Chestney; editing by Jason Neely)