TalkTalk's data breach last year appears to have pushed many of its customers towards BT.
According to insight consultancy Kantar Worldpanel, TalkTalk's market share is 8.3 per cent lower than it was last year.
By contrast, BT's share of the market has gone up by 7.8 per cent during this time.
Fiona Keenan, Strategic Insight Director at Kantar Worldpanel, commented: "In times of uncertainty, consumers tend to favour brands they trust.
"BT benefited more than anyone else from TalkTalk’s data hacking scandal last year, with 40 per cent of those leaving TalkTalk moving to BT in the months following on from the data breach."
Ms Keenan pointed out that while heavy promotional activity helped TalkTalk's market share grow in the first quarter of 2016, it remains about half of what it was this time last year.
"TalkTalk is a shadow of its former self compared with how well it was performing before the data hacking scandal came into play," she continued.
Ms Keenan said the momentum it generated following the initial period of recovery after the data breach has disappeared, with nearly a fifth of its customers wanting to leave as soon as they can.
"TalkTalk has its work cut out if it’s to see its previous success again," she stated.
TalkTalk has taken various steps to improve data security and offer assurances to customers on the issue.
For instance, in March this year, it became the first telecoms provider to use voice biometric technology to enable customers to access their accounts over the phone.
The technology allows them to use their voice to prove their identity, which means they no longer have to provide personal details to demonstrate they are who they say they are.