Airline pilots say a "gold standard" on how sky marshals should be used on UK flights is to be agreed with ministers.
Critics of sky marshals say security should be tight on the ground
Jim McAuslan, general secretary of pilots union Balpa, was speaking after meeting Transport Secretary Alistair Darling about the controversial plans.
He said he was "comforted and reassured" by Mr Darling's attitude and a protocol on use of marshals would be agreed in coming days.
It was also agreed that there should be more "joined-up thinking" on security.
Mr McAuslan told BBC News the rules were designed to ensure the "safest deployment possible" of marshals.
"We are against it in principle but if it is to be happening we want to have a proper protocol for their deployment.
"What we heard from the minister was reassuring: that if there is an identified credible risk, the airplane will not fly; that the commander of the aircraft remains in command; and that the commander will know that the police sky marshals have been deployed."
However, the union stressed it could "take some time" for a protocol to be fully agreed and until it was, Balpa was advising pilots not to fly with sky marshals on board.
Alistair Darling: Questioned by MPs
Mr Darling called the talks "extremely productive".
In the Commons on Tuesday he defended the use of sky marshals as a "responsible and prudent" security measure.
The transport secretary would not say whether any had yet been deployed, but promised the issue would be kept under review.
Mr Darling continued: "There is an increased threat and we have to deal with that in a balanced and proportionate way."
He said the government would keep people informed as much as possible, without providing a running commentary.
"Nor can the government get into a position where it is starting to give information to the very people we are concerned about, not only what we do know but what we possibly do not yet know."
Shadow transport secretary Theresa May told BBC Radio 4's Today programme she agreed with the principle of sky marshals but was unhappy that the proposals were announced without consultations with pilots and airlines being completed.
"The government has had a year to look at this and yet we are still in the position of not having the full operational procedures agreed," she said.
"These are all the details that are now being clarified. It's just a pity they weren't clarified before the government announced the use of sky marshals so we hadn't had 10 days of this debate and uncertainty."
The US has insisted the guards are put on certain flights, which would not otherwise be allowed to fly to or from its airports, or to enter its airspace.
But Portugal, Denmark and Sweden have all rejected the idea of having armed guards in the air.
Balpa, which represents more than 8,000 of Britain's 9,200 commercial pilots, has so far failed to negotiate a deal on air marshals with British Airways but has reached an agreement with Virgin.
Thomas Cook Airlines, which operates to Florida from Gatwick, Manchester, Newcastle and Glasgow airports, has said it will cancel flights if a marshal presented himself.