A spokesman said the first Question Time-style debate, being held at Brunel University on Thursday (March 19), clashed with a People's Question Time session the mayor was legally obligated to attend.
The Union of Brunel Students, which has organised the event, said it had been contacting Mr Johnson for months, but only received confirmation on Monday (March 16).
The union confirmed it intended to leave an empty chair in his place.
In a statement, the union added: “This is a long planned for event, which we hope to be well attended by both Brunel students and the local community alike.
"The students union is keen to encourage our members to register to vote (which is closing on April 20) and educate themselves on the forthcoming General Election, hence the timing of this hustings.
“We understand that it unfortunately clashes with a mayoral commitment in Mr Johnson's diary, and that he is therefore unable to attend.”
The 90-minute panel discussion will allow voters to question candidates ahead of the General Election in May.
'Got it in the bag?'
Former Ruislip Manor councillor Mr Cox said: “We've all got prior commitments, but personally I freed my diary.
“He clearly thinks he doesn't need to do more than one hustings because he thinks he's got it in the bag.
“He's taking the people of Uxbridge for granted and I hope he pays the price.”
Mr Lee said Mr Johnson was “running scared”, adding: “I think he's taking a leaf out of the Prime Minister's book, where he was using every single conceivable excuse not to have any one-on-one TV debates.”
Mr Summers said Mr Johnson was "taking voters for granted and treating them with contempt", adding: "I am glad to see the organisers of the Brunel hustings have decided to go ahead with an empty podium.
"Mr Johnson is running away from public debate on the issues, just like David Cameron is running away from a national TV debate."
Mr Duffin said: "Boris again shows his utter contempt and disinterest in Uxbridge. Not only does he want to remain mayor for a year but now he will only do one visit that isn't a stage managed PR show."
Further hustings will be held at St Margaret's Church, in Windsor Street, Uxbridge, on April 10 and Yiewsley Baptist Church, in Colham Avenue, Yiewsley, on April 26.
Mr Johnson plans to attend the former event, where organisers say “there will be no debate between candidates or with members of the public,” but not the latter.
A poster for the third and final hustings on April 26 says Mr Johnson "declined the invitation to participate".
Mr Johnson's spokesman had been unable to confirm the mayor's plans regarding the event and getwestlondon was unable to reach Uxbridge & South Ruislip Conservative Association for clarification.
To book tickets for the hustings at Brunel University, in Kingston Lane, Uxbridge, click here.