Tory MPs have said the King “should not be surprised” if any comments alluding to the handover of the British territory incites a walkout.
It comes in the wake of an ongoing row between the UK and Spain over sovereignty.
The visit has already sparked controversy, with Gibraltarians calling for the trip to be cancelled.
Tory MP Andrew Rosindell has predicted “serious anger” if the King tried to “re-open the non-negotiable issue of Gibraltar’s sovereignty”.
He said: “The people of Gibraltar are rightly nervous. They break the rules while we, in turn, welcome the King and Queen of Spain for a state visit. What kind of message is that for the people of Gibraltar?”
Britain’s Ambassador to Madrid has said it’s likely that King Felipe will bring up Gibraltar when he makes his speech at Westminster.
It’s all the more likely after his request last year for a negotiation of a handover to Spain.
The royal visit has been postponed twice – once due political reasons in Spain and the other time because of a clash with the UK snap election.