Firefighters in London and Manchester will assist people responding to Maruading Terrorist Attacks (MTAs).
The Fire Brigades Union said the move would help “protect the public and firefighters”.
MTAs are defined as terrorist incidents when the attacker moves through a location, attempting to find and kill or injure as many people as possible.
In the plans, firefighters would be expected to assist with the evacuation of casualties as well as performing their firefighting duties.
Firefighters will also be provided with appropriate training and equipment, including ballistic helmets and goggles.
Greater Manchester Fire and Rescue Service was criticised for delays in reaching the aftermath of the 2017 attack at the Manchester Arena.
Crews took two hours to reach the site.
Twenty-two people died and hundreds more were injured when Salman Abedi detonated a suicide bomb in the foyer of the arena on 22 May 2017 where popstar Ariana Grande was performing.
Matt Wrack, Fire Brigades Union general secretary, said: “Our members go to work to save lives and will want to do so wherever they can, no matter how dire or threatening the circumstances.
“MTAs are a horrendous prospect, and any firefighter that can help as part of a properly-organised response, where risk is appropriately mitigated, would want to do so.”
The agreements were made following negotiations by the Fire Brigades Union and ballots of London and Manchester firefighters earlier in the year.
Mr Wrack said the negotiated terms “recognise the increased risks and responsibilities”, adding that the union was “keen to reach a similar agreement on all-UK basis”.
He added Manchester and London would now be “the only two fire and rescue services in the UK which will have MTA-trained firefighters at every station in their area”.
London Fire Commissioner Andy Roe said the plans “strengthen the brigade’s response to terror attacks and ensures that our firefighters will be better able to help people when they attend the scene, providing immediate assistance”.