* Acea board delayed due to GDF doubts on plan, debt-source
* Main shareholder says has full trust in Acea management
* Acea shares up 0.51 percent
(Adds detail, background, share price)
ROME/MILAN, Oct 16 (Reuters) - Acea's board meeting has been postponed to next week due to its French partner GDF Suez's doubts about the Italian utility's business plan and debt levels, a source close to the Rome company said on Friday.
Acea said in a statement on Thursday that it had adjourned its board meeting -- which was to approve its new business plan -- to Oct. 21 due to the number of issues that needed to be addressed.
"The French expressed doubts on the plan and in order not to go ahead without their approval, it was decided to examine things more thoroughly until next Wednesday," the source said.
GDF and Acea have an electricity joint venture and the French utility, which has 9.98 percent of Acea, is keen to extend this to include natural gas activities.
Rome Mayor Gianni Alemanno said on Thursday there currently were no plans to extend the joint venture to the gas sector. The city of Rome owns 51 percent of Acea.
Acea's new business plan would be a stop gap measure ahead of a longer term strategy, Alemanno said on Friday.
"I have full trust in the management of Acea," he said.
Acea's debt rose to 1.978 billion euros at the end of June from 1.633 billion euros at the end of 2008.
At 1423 GMT, Acea shares were up 0.51 percent at 8.79 euros while the DJ Stoxx utilities index was 0.86 percent lower. (Reporting by Stefano Bernabei, writing by Stephen Jewkes; editing by Karen Foster)