Undercover armed air marshals are to be introduced to some UK flights for the first time early this week.
Undercover air marshals will be employed on some flights
They have all been trained by the Metropolitan Police, and have a police or military background.
The move was a "responsible and prudent step" in response to the heightened state of terror alert in the US, Home Secretary David Blunkett said.
But the plan is opposed by the British Airline Pilots Association, which says many pilots may refuse to fly.
Balpa general secretary Jim McAuslan said: "For a start we don't believe that guns and air travel mix."
He said marshals could only be used under certain conditions, including that the pilot knew when one was aboard.
"The commander needs to retain authority over the whole of the aircraft."
The air marshals will be used on a small number of transatlantic flights where it was deemed right for the situation, BBC correspondent Margaret Gilmore said.
British Airways and Virgin Atlantic have declined to comment.
The announcement follows a number of security scares involving airports in recent days.
On Wednesday six Air France flights between Paris and Los Angeles were cancelled amid fears of a terror attack.
In a separate warning on Sunday, the government said it believed terrorists could be in the final stages of planning an attack in Saudi Arabia.
British nationals have already been warned against all but essential travel to the kingdom following attacks in Riyadh in May and November this year.
A joint statement issued by the home secretary and Transport Secretary Alistair Darling said it was still safe for the public to fly and that the increase in security levels in the US in recent days meant the UK should propose a "proportionate and appropriate level of response".
Mr Blunkett said the police and security services were already operating at the highest levels.
"I can assure the travelling public that if we believed it was not safe for them to travel or fly we would say so. What we are proposing are some sensible additional security measures."
The French prime minister ordered flights between Charles de Gaulle airport and Los Angeles to be cancelled on Wednesday and Thursday after US intelligence suggested al-Qaeda members posing a "credible threat" were due to board.
And hours later, the Delta Airlines terminal at New York's LaGuardia Airport was evacuated in a scare.
Mr Darling announced back in February that armed undercover police had been trained for use on UK passenger flights.
At the time airlines raised concerns about the prospect of guns being carried on board planes.
The Conservatives welcomed the move, but said it should have been made earlier.
Patrick Mercer, shadow minister for homeland security, said: "I would have liked to have seen it at least a year ago, if not more than
Sunday's announcement follows similar moves by the Australian Government to use armed guards on many flights between Australia and Singapore.
A deal between the Australian Government and national airline Qantas to use sky marshals was announced on Friday and there are also plans to allow Singaporean security officers to guard flights between the two countries.
Air marshals have been used in America since before the 11 September terror attacks on New York when there were just 33 sky marshals, flying mainly on international flights.
Since then the programme has been greatly expanded in response to the attacks, although the exact number of marshals is classified.
Captain Dennis Breslin, an American Airlines pilot, said he would not fly without their protection.
"Because between the door of the cockpit and the cabin, if the door is breached and those pilots are taken out as they were on 9/11, you're all doomed," he told the BBC.