Darren Johnson posted on Twitter after trying to find a licensed taxi
The London Assembly chairman who put messages on Twitter asking for "an illegal" minicab has apologised.
Green Party member Darren Johnson posted the comments on Thursday night after being unable to find a licensed taxi after a City Hall party.
On Friday he said: "I'm very sorry that I stupidly took an illegal minicab".
But Steve MacNamara from the Licensed Taxi Drivers' Association said Mr Johnson's actions were "outrageous" and called for him to step down.
Mr Johnson's apology comes during a advertising campaign warning Londoners about the dangers of unlicensed minicabs, which was launched in November.
"I was lucky that it did not cost me much more in terms of my personal safety," said Mr Johnson.
"Londoners should always heed the official advice to only ever take a licensed taxi cab or pre-booked licensed minicabs to get home," he added.
"They should learn from my mistake that unlicensed minicabs are illegal and expensive and place personal safety at serious risk."
Mr MacNamara said: "Not only did he do something incredibly stupid by getting by in an unlicensed cab. He compounded that stupidity by bragging about it.
"Every month a large number of robberies and assaults take place in unlicensed vehicles.
"The message it sends out to Londoners is that its perfectly acceptable to use an unlicensed cab. Of course it isn't," he added.
"He now thinks its acceptable to apologise for it and think that it will all be forgotten. He's an embarrassment to the GLA, to the mayor and to Londoners. He should step down."