The carrier will run up to 40 flights a week between the US and Heathrow from August, including a second daily service from Washington DC and a daily service from Houston.
Additional London services are due to resume in the coming months, as well as a new non-stop route from Boston to London.
The expansion comes after the UK government confirmed fully vaccinated EU and US travellers will not have to quarantine upon arrival in England from 4am on Monday (2 August).
Scotland and Wales have introduced the same rules.
Patrick Quayle, vice-president of international network and alliances at United, said Wednesday’s (28 July) announcement was "yet another major milestone in recovering from the Covid-19 pandemic".
"United has demonstrated that we can operate flights between the US and England safely and we are eager to help rebuild these economies by facilitating business and leisure travel," he added.
