A £45,572 licence fee-funded party at a top venue was "the most cost-effective way possible" to launch a television drama series, the BBC has insisted.
Bosses came under fire for spending so much cash on entertaining 300-plus people at the Oxo Tower in London to promote Merlin at a time of staff cutbacks.
It was the biggest of four such events this year at a total cost of nearly £164,000, Channel 4 News established using freedom of information laws.
But the Corporation's chief operating officer Caroline Thomson defended the bill.
"We don't party for party's sake; we choose the programmes we are going to launch at parties very carefully. We only did three BBC1 dramas last year," she told the programme.
"When we do it, we do it because we are very proud of the programmes and we therefore do the launch in the most cost-effective way possible."
The cost included food, drink, room hire, photography, security, entertainment, invitations, transport and production.
Other programmes launched with parties were The Passion, No 1 Ladies Detective Agency and Little Dorrit.
Gerry Morrissey, general secretary of broadcasting and media union Bectu, said he was shocked by the spending as the BBC had plenty of its own facilities to host such an event.
"I am shocked, because at a time when the BBC has made 2,000 staff redundant this year... it will seem extremely strange to people inside the BBC that this level of money is being spent externally on a launch party," he told Channel 4 News.
