Boris Johnson had a third meeting with the Murdochs that was not declared on City Hall's hospitality register, leading to accusations that he attempted to conceal his close relationship with the media baron.

He had what is described as "a working lunch with James Murdoch and Rebekah Brooks" on 20 May 2011, just as the phone-hacking scandal was escalating.

Labour has already called for an investigation into the London mayor after it emerged that Johnson met Rupert Murdoch at his home in London on 24 January 2011 and that he also planned to meet Rebekah Brooks and Will Lewis on 14 January that year, although Lewis had to pull out of the meeting at the last minute.

City Hall says the lunch with James Murdoch was not a secret and had been disclosed on the Greater London Authority website for "nearly a year" but appeared to have gone unreported until today.

A spokeswoman directed inquiries to a freedom of information section on the website that shows the meeting was made public in July 2011. However, it is not on the hospitality register.

She said City Hall wasn't obliged to declare the meeting because the monetary value of the lunch did not meet the threshold of £25 and that it was, in any case, "commercially sensitive".

The meetings came to light after a Freedom of Information request on the hospitality register by the left-leaning political website www.politicalscrapbook.net.

It questioned City Hall's response, asking: "Where does James Murdoch eat for less than £25 a head?"

Rupert Murdoch, who is currently holidaying in Croatia, tweeted politicalscrapbook after its first story appeared yesterday: "@PSbook Really? Actually just reading his new book on London. Writes like a dream."

Len Duvall, leader of the Labour group on the London Assembly, called for City Hall to investigate the "truly scandalous" dinners.

"This is extremely serious, for the mayor to not declare a meeting with Rupert Murdoch at the height of the phone hacking crisis is truly scandalous," he said.

• This article was amended on 22 June 2012. The original said Boris Johnson met Rebekah Brooks and Will Lewis for a lunch on 14 January 2011, which is what was suggested by a document released by the mayor's office in response to a freedom of information request. The mayor's office has subsequently clarified that while Lewis was scheduled to attend the lunch, he had to pull out at the last minute and did not attend.