Power failures caused delays and disruption on London’s Tube network on Monday, as several of its busiest lines were suspended.

The Bakerloo line was completely suspended as of 5pm local time (7pm Cyprus time), while other services including the Jubilee, Elizabeth and Piccadilly lines were facing severe or minor delays following a power outage roughly two hours earlier.

The potential for power failures to cause major disruption came into sharp focus in March when London’s Heathrow airport was forced to shut for 18 hours due to a fire at a nearby electrical substation, stranding hundreds of thousands of air travelers.

TfL said via phone that most Tube lines would return to normal service shortly, without giving a specific timeframe.

Transport trade union TSSA called for an inquiry into the incident to ensure a similar power outage does not happen again.

“Safety for all is key at this moment, but in due course we will need a proper inquiry into what happened and the lessons to be learned,” TSSA secretary-general Maryam Eslamdoust said.