The UK's biggest telecoms network is facing a class action lawsuit over claims it failed to compensate more than 2.3million customers who were overcharged for eight years.
In 2017, industry watchdog Ofcom said that BT had unfairly charged millions of landline customers since 2009.
It said that despite a 25% fall in wholesale costs, major landline providers in the UK had increased line rental by 28-41%.
Ofcom strongly criticised market leader BT for raising prices, saying that customers were being given "poor value" for money.
As a result of the review, the firm reduced the price of its landlines by £7 a month.
However, lawyers say that millions of loyal customers have still not been compensated for the historical charges.
And while the law does not allow you to backdate claims for up to eight years, lawyers can claim justice for 2015 onwards.
"Ofcom made it very clear that BT had spent years overcharging landline customers, but did not order it to repay the money it made from this," said Justin Le Patourel, founder of consumer group Collective Action on Landlines (CALL) and a telecoms consultant who worked for Ofcom for 13 years.
The claim could result in payments of up to £500 each for 2.3million BT customers, should it be successful.
"We think millions of BT's most loyal landline customers could be entitled to compensation of up to £500 each, and the filing of this claim starts that process."
BT said it "strongly disagrees" with the claim that it had engaged in anti-competitive behaviour and intends to defend itself "vigorously" in court.
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A spokesman for BT said: "We take our responsibilities to older and more vulnerable customers very seriously and will defend ourselves against any claim that suggests otherwise.
"For many years we've offered discounted landline and broadband packages in what is a competitive market with competing options available, and we take pride in our work with elderly and vulnerable groups, as well as our work on the Customer Fairness agenda."
Who is affected?
The case represents customers who purchased a BT landline contract, but did not also take BT broadband or pay TV.
These customers were, according to Ofcom, more likely to be old, on low incomes and vulnerable.
Ofgem said that many of the affected customers had "been with BT for decades" and were more likely to be old, on low incomes and vulnerable.
As a result of Ofcom's review, BT announced that it would slash its landline prices by £84 a year.
However the telephone giant did not make efforts to repay customers for the previous eight years of overcharging.
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Le Patourel is also seeking compensation for customers who took both a broadband service and a BT landline, but not together as a package (or 'bundle'). These people were excluded from BT's 2017 price cut, and so continue to be overcharged to this day.
Natasha Pearman of Mishcon de Reya who is representing Le Patourel and CALL, said: "This is a specialist claim that will be heard before the Competition Appeal Tribunal.
"It is a classic example of a loyalty penalty, which were the subject of a super complaint by Citizens Advice, due to their harmful effects on consumers.
"It will take time to gather evidence and bring it to trial, but we are very confident that eventually millions of BT's most loyal customers – many of whom are older and potentially vulnerable – will receive a significant rebate".
