LONDON, July 24 (Reuters) - Trying to spot whether the Libor interest rate was being manipulated by tiny amounts would be "prohibitively expensive" and intrusive, Britain's financial watchdog said on Tuesday.

Adair Turner, chairman of Britain's Financial Services Authority, said there was a debate to be had on whether regulators should have spotted the rigging of Libor during the 2007-09 financial crisis.

But spotting the rigging which led to tiny changes in earlier years would have been more difficult, Turner said.

"In relation to the earlier period, to the manipulation of rates by a minute amount for a short period in either direction, I do not believe these problems could have been spotted from outside except via supervision so intensive as to be prohibitively expensive," Turner told a Bloomberg News event.

"Alongside more intense and better focused supervision, therefore, robust after-the-event enforcement action and sanction will have to remain a key regulatory tool," Turner added.

Alongside both, we also need strong management commitment to better culture and better values, he said.

UK bank Barclays was fined a record sum for rigging Libor with other banks also being investigated by the authorities in Britain, the United States and elsewhere.