The Upper Room comprises 13 paintings of rhesus monkeys

The body, which regulates charities in England and Wales, said it was "very disappointed" gallery bosses had not asked its permission first.

The artworks include Chris Ofili's The Upper Room, bought for �600,000, and two pieces by sculptor Bill Woodrow.

Tate director Sir Nicholas Serota has accepted the criticisms and attributed the error to "a general oversight".

"It appears that no one at the Tate was aware that we should have been seeking permission of the Charity Commission," he said.

'Serious shortcomings'

The Upper Room - a series of 13 paintings of rhesus monkeys displayed on elephant dung supports - was bought in 2004 while Ofili was on the Tate's board of trustees.

Charity Commission chief executive Andrew Hind said its investigation into the sale had revealed "serious shortcomings".

"In any charity we would be concerned that such basic matters were neglected," he continued.

"But in a charity of the size and stature of the Tate we are very disappointed."

The commission ruled the Tate could keep the paintings but should ask for permission before buying any further works from its serving trustees.