Sebastian Vettel has hit out at the Imola stewards, branding the delay in imposing a time penalty for a pre-race misdemeanour "unprofessional".

It was another weekend to forget for Sebastian Vettel.

Qualifying thirteenth, three places behind his teammate, the German headed into Sunday's race feeling more confident than he did in Bahrain.

However, ahead of the start both cars suffered issues with their rear brakes, and while Lance Stroll was able to start from the grid, as work continued on Vettel's car the German was forced to start from the pitlane.

Having worked his way up to 14th, the Aston Martin driver pitted at the end of lap 20, being the first driver to switch to slicks.

However, as he exited the pits he was informed that he had incurred a 10s stop and go penalty for not having his tyres fitted within the mandatory 5-minute window ahead of the start of the race.

Consequently he had to pit to serve the penalty at the end of lap 22.

Working his way up to 13th again, the German was told to retire the car just a few laps from the end, his team fearing a "loose gearbox".

At the end of a difficult afternoon it was the late timing of the penalty that most frustrated the four-time world champion.

"Obviously the guys tried everything on the grid and I think they did really well," he told Sky Sports. "So they were really alert.

"But I think we could have had a better race if the FIA was more alert," he added.

"We broke a rule, that's why we got a penalty," he admitted. "But they didn't bother until way into the race. So by that time the penalty cost a lot more than it would have earlier in the race.

"So that's not very professional. But it certainly wasn't a decider for us today."

"I don't know about taking longer than it should have," responded race director, Michael Masi. "It was obviously reported by the technical delegate.

"I'd have to have a look at the reports but once it was reported by the technical delegate, being the paper report that appears in the document management system, at that point the stewards had a look at the regulations, confirmed the evidence and determined what the penalty was."

Check out our Sunday gallery from Imola, here.