A spokesperson for the GLA said the monitoring officer reviewed the complaint against the mayor and would not be investigating three of the four allegations.
It added, an investigation will now take place to establish if the mayor “exercised an appropriate level of caution” in deciding to accept the tickets.
Ms Hall said: “Whilst I welcome an investigation into the propriety of accepting these tickets, after months of asking Khan for answers, I am disappointed that the monitoring officer isn’t considering the fact that the mayor declared these tickets late, initially mis-declared who donated them, that they were undervalued, or that the donor was a GLA contractor.”
“More answers are desperately needed,” she added.
A spokesperson for Sir Sadiq said: “The mayor has no involvement in the procurement process for GLA events, nor in the tendering of these contracts.
“The mayor’s office will continue to ensure all the right processes are followed, and looks forward to explaining the approach that was taken in this instance.”
