The prosecution was brought by Transport for London (TfL) for offences committed between January and March 2016.
At Westminster Magistrates Court last week BT pleaded guilty to: unsafe execution of street works on Brixton Road; failure to serve necessary statutory works notices following the completion of work on Clapham Road; failure to pay the fixed penalty notices originally issued by TfL in response to the Clapham Road offence.
It was fined a total of £4,500 and ordered to pay £3,916 in prosecution costs.
BT has now been fined £32,020 for poorly managed roadworks since 2010. TfL has prosecuted BT for 37 previous offences since 2010 and issued more than 800 fixed penalty notices of which 200 were issued within the last year.
The second worst roadworks offender in London is Thames Water, with 13 prosecutions against it, then Infocus with 11 and Virgin Media with 10.
In passing sentence against BT, the judge said: " I don't think… even the most basic steps were taken to put proper measures in place to protect the public. I hope lessons are being learnt."
Garrett Emmerson, TfL chief operating officer for surface transport, said: "BT has become London's worst roadworks offender, repeatedly disrupting Londoners with poorly planned, badly managed and sometimes dangerous roadworks. We are committed to keeping London's roads as clear as possible preventing unnecessary traffic build up, which disrupts people's daily commute and worsens air quality. We will continue to push for the toughest penalties possible for utility companies caught acting unlawfully and are pleased that BT themselves have recognised the need to improve."