By Elias Biryabarema and Richard Lough ENTEBBE, Uganda (Reuters) - A ceremony to formally end Congo's bloodiest conflict in a decade was delayed on Monday by an 11th-hour spat between the government and M23 rebels over whether the document should be entitled a peace agreement or a vaguer declaration. M23 gave up their 20-month insurgency last week after the Democratic Republic of Congo's army, backed by a U.N. force, routed them from their last hilltop hideouts along the eastern border with Rwanda and Uganda. The deal between the Kinshasa government and M23 was due to be signed in the Ugandan city of Entebbe at 6 p.m. (10 a.m. ET) but suddenly delayed, with no one knowing whether the hold-up would last for hours or days.