The prosecution was brought by Transport for London, which is staging a crackdown on unsafe working and unnecessary delays and disruption caused by utility companies.
Virgin Media pleaded guilty to two offences committed on Chertsey Road, Twickenham in February 2014.
These included carrying out unsafe working practices and breaching permit conditions, namely excavating the entire width of Chertsey Road.
Virgin Media’s contractor, McNicholas, also pleaded guilty to a breach of permit conditions.
Westminster Magistrate’s Court fined the firms over £6,500 and legal costs of £5,040.
It is the latest in a series of successful prosecutions by TfL as it works to reduce unnecessary roadworks to improve traffic flow and conditions for all London’s road users.
Earlier this month, TfL successfully prosecuted Virgin Media for five offences on Croydon Road in the London Borough of Sutton, including unsafe working practices and permit breaches.
Leon Daniels, Managing Director of Surface Transport at TfL, said: “In a frustrating case of déjà vu, this is the second successful prosecution of Virgin Media in as many months.
“Utility companies must ensure that any disruption caused by roadworks is minimised and the most stringent safety is upheld. As we have demonstrated time and time again, we will prosecute those who flout this.”
Last year, TfL successfully prosecuted Thames Water for nine road works offences and Cable & Wireless for one offence of working in breach of permit conditions and causing significant road disruption as a result.