Premier League clubs have made limited progress on improving access for disabled fans, campaigners have said.
Thirteen out of the 20 sides are failing to provide the required number of wheelchair spaces, says the Equality and Human Rights Commission (EHRC).
It says only seven clubs have larger, fully equipped toilets, while seven clubs are breaking Premier League rules on providing information to fans.
The Premier League said clubs were working hard to improve facilities.
A BBC report in 2014 found that 17 of 20 clubs did not provide enough wheelchair spaces.
Clubs later set a self-imposed deadline to meet standards by August 2017 and the Premier League has pledged to publish a report then to highlight the work carried out.
EHRC chair David Isaac said it would launch an investigation into clubs who had failed to meet the minimum requirements and did not publish a clear action plan or timetable for improvement.
"The end of the season is fast approaching and time is running out for clubs," he said.
"The information we received from some clubs was of an appalling standard, with data missing and with insufficient detail. What is clear is that very few clubs are doing the minimum to meet the needs of disabled supporters.
"The Premier League itself does not escape blame. They need to make the concerns of disabled fans a priority and start enforcing their own rule book. We will be meeting individual clubs and asking them to explain themselves and tell us what their plans are."
Clare Lucas, activism manager for learning disability charity Mencap, said clubs should have 'changing places' toilet facilities, with more space and equipment including a height-adjustable changing bench and a hoist.
"For too long Premier League clubs have neglected the needs of their disabled fans, many of whom are forced to be changed on toilet floors, because clubs are yet to install proper facilities. It is simply inexcusable," she said.
What the commission says
According to the EHRC, the following clubs have not met requirements in particular areas:
Wheelchair spaces: Failing to provide the minimum number - Arsenal, Burnley, Chelsea, Crystal Palace, Everton, Hull, Liverpool, Man Utd, Stoke, Sunderland, Tottenham, Watford, West Brom
Toilets: Without larger, fully equipped toilets, known as 'changing places' toilets - Bournemouth, Burnley, Chelsea, Crystal Palace, Everton, Hull, Middlesbrough, Stoke, Sunderland, Swansea, Tottenham, Watford, West Brom
Information: Not publishing access statements to give disabled fans information about their ground - Burnley, Crystal Palace, Hull, Man Utd, Middlesbrough, Stoke, West Ham
What the Premier League says
"In September 2015 Premier League clubs unanimously agreed to improve their disabled access provisions by meeting the Accessible Stadia Guide (ASG) by August 2017.
"Clubs are working hard to improve their facilities and rapid progress has been made. The improvements undertaken are unprecedented in scope, scale and timing by any group of sports grounds or other entertainment venues in the UK.
"Given the differing ages and nature of facilities, some clubs have faced significant built environment challenges. For those clubs cost is not the determining factor.
"They have worked, and in some cases continue to work, through issues relating to planning, how to deal with new stadium development plans, how to best manage fan disruption or, where clubs don't own their own grounds, dealing with third parties.
"Clubs will continue to engage with their disabled fans and enhance their provisions in the coming months, years and beyond."
The story so far
2014: A BBC investigation finds that 17 of the 20 clubs in the top flight at that time had failed to provide enough wheelchair spaces.
September 2015: The Premier League promises to improve stadium facilities for disabled fans, stating that clubs would comply with official guidance by August 2017.
September 2016: Campaigners say up to a third of clubs will miss the deadline to meet basic access standards.
October 2016: Leading disability campaigner Lord Holmes tells MPs that legal action against clubs and the Premier League remains an option if standards are not met.
January 2017: A report by MPs says some clubs could face sanctions because they are not doing enough. Manchester United,Liverpool and Everton announce plans to develop their grounds to accommodate more disabled supporters.
February 2017: A Premier League report outlines the detailed work the clubs are undertaking to make sure they meet guidelines but adds that at least three clubs will miss the August 2017 target.
April 2017: Premier League clubs have made limited progress on improving access for disabled fans, says the Equality and Human Rights Commission.