A Metro spokeswoman said the fault was at the point where Frankston, Cranbourne and Pakenham trains leave the City Loop and meant the signals in one of the Loops' four tunnels were not working.

Six trains between Richmond and Southern Cross Station were stuck as a result and passengers were forced to cancel meetings and reschedule their mornings. Some trains were able to move slowly to Parliament station were passengers disembarked.

The fault was fixed about 9.35am and trains on the effected lines again began running through the Loop. Metro is investigating the cause of the fault.

Age journalist Henrietta Cook was stuck on a city-bound Pakenham train just outside the Loop from about 9.10am.

The driver made an announcement that a power outage meant signals were not working in the City Loop, with three trains waiting ahead.

At 9.35am another announcement was made that the trains ahead would start moving but it would be a slow process.

Metro warned passengers to expect residual delays of up to 15 minutes.