The commission's investigation was prompted by calls from journalists asking why Labour's 2015 general election return, published in January, did not include the stone carving, which was widely mocked after being unveiled. It turned out that failing to declare two payments totalling £7,614 relating to the tablet was not the only mistake in Labour's election expenses return. These two payments were among £123,748 of payments missing from Labour's 2015 election return.
A further 33 receipts, worth £34,392, were missing, the commission said.
The £20,000 is the highest fine it has imposed since it was formed in 2001.