The response to major incidents such as the Grenfell Tower fire has become a "postcode lottery", says the Fire Brigades Union.
Crew levels and equipment for responding to tower block fires vary greatly across the country, according to research by the union.
The "pre-determined attendance" (PDA) for a fire - or the number of fire engines that should automatically be sent to a fire or other incident - vary according to location.
For example, the FBU said that that the PDA in Bedfordshire is four fire engines and one aerial platform (vehicles with long ladders or platforms needed to deal with fires in high buildings).
Hampshire, on the other hand, has a PDA of eight fire engines and an aerial vehicle.
The FBU said that, although there are 125 of these ladders and platforms in England, just 33 of them are available all day every day, due to lack of firefighters.
Matt Wrack, general secretary of the FBU, said in a letter to the Prime Minister that, following the deaths of at least 80 people at Grenfell Tower last month, firefighters had hoped for "an immediate response" on the issue from the Government.
The fact this has not happened is cause for "concern and alarm", the FBU says.
Mr Wrack said the findings of the research were "extremely concerning".
He added: "In the light of the terrible tragedy at Grenfell Tower, this situation is utterly unacceptable.
"We find it staggering that nothing has been done to address this grossly unjust postcode lottery of resources, and the fact that governments in all parts of the UK appear not to have even considered it is a disgrace.
"They now need to urgently instruct fire services to improve their fire and rescue planning to ensure a full and professional response to such incidents all over the UK.
"Citizens everywhere need to feel safe and confident that those in authority are taking their safety seriously.
"Anything less is, frankly, obscene."