An attempt by Yarra Trams to reduce travel times by introducing a speedy new timetable has been abandoned after it backfired and took the company to the brink of a costly failure to meet its punctuality targets.

The "seasonal" timetable, which slashed scheduled journey times across the network from December, was quietly dumped at the end of March, the single worst month for tram punctuality since Keolis Downer took over running the system in 2009.

About one in three trams ran late on average on the worst performing lines, mostly in Melbourne's south-east and on the Swanston Street-St Kilda Road corridor.

Yarra Trams' performance has turned 180 degrees in recent months, from record on-time results in mid-2014 to an all-time low last month.