Boris Johnson has been called before the London Assembly to give evidence on the failed Garden Bridge project.
The assembly has used legal powers to demand his attendance - the first time such measures have been used to require a London former mayor to appear.
Mr Johnson declined to speak to an earlier inquiry into the project.
A spokesperson for Mr Johnson dismissed the new hearings as "pointless" and criticised Mr Khan for focusing on "issues of the past".
Len Duvall AM, chair of the Greater London Authority oversight committee said: "When Margaret Hodge appeared before us and explained that Boris Johnson did not participate in her review of the Garden Bridge Project, we realised that an important part of the story was not being made public.
"Dame Margaret did not have the power to formally request the former mayor to give evidence, however the assembly does have that statutory power.
"So I think it's only right that Boris gives us his side of the story. I know that Londoners are still very interested to know how the whole project got so far down the track, before the plug was pulled."
Refusing to attend can be considered a criminal offence. To date, no-one has refused a summons from the assembly.
A spokesperson for Mr Johnson said: "Instead of improving the lives of ordinary Londoners, Sadiq Khan and his team have wasted 18 months filing a series of pointless reports on his illustrious predecessor simply to fill column inches and distract from his own lack of vision and delivery.
"It's no surprise then that Khan's team aims to fill the void by calling on the previous Mayor to give evidence on the Garden Bridge - a subject Labour members have questioned him on multiple times when he was still the mayor," the spokesperson said.
"In Sadiq Khan London is trapped with a selfie mayor using his position to grandstand on issues of the past when he should be delivering a positive vision for the greatest city on earth."