Boris Johnson has backed out on a gentleman's promise he made to kick-off a high profile anti-airport expansion protest this month.

London's new Mayor - who before he was elected said he is "implacably opposed to a third runway" at Heathrow - had told organisers of the Make a Noise carnival that he would lead the rally on May 31.

More than 15,000 people are expected to attend the demonstration march from Hatton Cross to Sipson, the village north of Heathrow that would be razed if a third runway goes ahead.

Mr Johnson had informally agreed with organisers to make a speech and officially lead the two-and-a-half mile walk to Sipson.

But by this week he still had not fully committed to the event.

When pressed for an answer yesterday an environmental officer at the Mayor's office said Mr Johnson is unable to attend "due to diary commitments".

She said: "In his absence deputy Mayor Richard Barnes will join the carnival of marchers and demonstrate his and the mayor's opposition to expansion at Heathrow."

Campaigner Tamsin Omond said: "It's a real disappointment Boris can't be there because he's missing out on the biggest show of opposition to Heathrow expansion before Labour makes its announcement.

"During the mayoral campaign Boris told voters that he was against the third runway, it would have been a great confirmation of this campaign promise to have him at the front, leading the carnival procession."