Mental health charity Mind said it is "disgraceful" that anyone in the country should be forced to wait a year or more for their application to be processed.
Hospitals and care homes must apply to councils for Deprivation of Liberty Safeguards to make decisions for anyone thought to lack the mental capacity to do so themselves, such as those with dementia or serious mental health problems.
There is currently a legal maximum time limit of 21 days for applications to be processed.
But NHS Digital data shows Northumberland County Council completed 1,870 applications in 2020-21, with each one taking one year (373 days) on average.
This was up from an average of 257 days the year before.
Only around 14% of the 390 standard applications completed last year were processed within the target time.
And one application had taken over three years (1,366 days) to be finalised.
The average time to complete an application across England rose from 142 days in 2019-20, to 148 last year – with one in the London borough of Merton taking over 10 years.
Mind said DoLS were intended to protect some of the most at risk members of society, but they have been "riddled with problems" and left thousands of people without vital legal protection.
Alison Cobb, specialist policy advisor at the charity, said: "These delays mean that people are being deprived of liberty without the right safeguards, as providers cannot use all the protective legislation.
“It is a disgraceful breach of human rights that people are waiting a year or longer for their DoLS application to be processed.
"This can mean that they endure unnecessary delays to being placed in the best care setting for them, which would help to protect them from harm."
The Government has committed to replacing the safeguards with Liberty Protection Safeguards in April 2022, but Mind fears these will also fail to put individual needs at the heart of the process.
Of the 1,870 applications completed in Northumberland last year, the largest proportion (34%) were from nursing homes.
And those with applications submitted on their behalf were typically aged 85 and over – at a rate of 7,842 in every 100,000 people in that category.
The Alzheimer's Society said local authorities have been overwhelmed with applications since a 2014 court ruling widened the definition of deprivation of liberty.
Gavin Terry, head of policy at the Alzheimer’s Society, added: "As applications are piling up on desks, people with dementia are effectively being unlawfully deprived of their freedom, unable to come and go as they please, without any safeguards in place to protect them."
The Department of Health and Social Care said protections for people who need them will be improved and extended through the new LPS.
A spokeswoman added: “To protect the human rights of people who may lack mental capacity it is important care homes and hospitals continue – until the new safeguards are in place – to make DoLS applications and local authorities consider them.”