The popular TV show, which began its second series last night, charts the ups and downs of life in the Crawley family. The period drama takes place just before the beginning of the First World War.
However, web users logging on to the catch-up service after the programme had been broadcast were only able to view three adverts on a loop. ITV soon posted an apology on the website admitting is was experiencing "technical difficulties".
"We are investigating and apologise to those who were unable to use the service during this time," ITV told the Telegraph.
The outage, which last for several hours, lead web users to vent their frustrations on micro-blogging site Twitter. It isn't the first time ITV has failed to cope with demand from web users for streaming TV shows over the web. Last Year, the broadcaster suffered with problems when streaming the first match of the World Cup.
Earlier this year, ITV revealed it plans to offer bespoke content through the online TV service that web users will have to pay for. The broadcaster is to begin a trial of paid-for internet-only shows or 'webisodes' of its popular shows such as Coronation Street and The Only Way is Essex at the start of next year.