Thames Tideway has now assessed the impact of the lockdown and more restricted working practices on the vast project.
It has warned shareholders that the project will be pushed back nine months into the first half of 2025.
Project engineers now estimate that Covid-19 will add £233m to costs, taking the expected final project budget to £4.13bn.
Tideway is in discussions with regulator Ofwat over a package of measures to mitigate the financial impact of Covid-19 on the company.
“We are making progress in these discussions and we expect to reach a full agreement in the coming months,” said a spokeman.
Following a temporary pause of all but essential works at the start of the lockdown, construction work recommenced on the majority of Tideway’s worksites in May, but at a lower level of activity in order to comply with social distancing requirements.
The project is approaching 60% completion and still in peak construction.