Fire crews have now extinguished 17 of the 20 containers at the Buncefield oil depot despite concerns about the contents one container hindering progress overnight.
It is now hoped the blaze which has been raging for more than 48 hours could be out by lunchtime.
Hertfordshire Police Assistant Chief Constable Robert Beckley said there was "real optimism" that the inferno would be extinguished during the day, with some in the fire service confident it would be over before this afternoon.
Crews were focusing their attention on the three tanks which remained alight this morning, after pulling back briefly overnight when a "structural failure" at one was thought to pose a risk to others nearby.
They will tackle the biggest remaining tank at the depot last.
"There's real optimism the fire will be extinguished today," Mr Beckley told the BBC Radio 4 Today programme.
"It's hard to be definite on this but speaking to my colleagues in the fire service, one or two would stake their salaries that it could potentially be out by lunchtime, but I wouldn't want to hold them to that."
A Hertfordshire Police spokeswoman said: "It has been a demanding night but real progress now looks to have been made."
Overnight "cooling patrols" had been successful and the despite the "still spectacular" emissions from the site, the fire remained under control, she added.
But Hemel Hempstead Conservative MP Mike Penning called for a public inquiry into why the fire started, and whether residents should be living so close to such a depot.
The teams started work again last night after confirming that a volatile tank contained aviation fuel.
More than 150 firefighters have been battling to douse the blaze, which began following a series of explosions at 6am on Sunday.
But yesterday afternoon it was decided to withdraw crews amid fears they could be killed if a so-far-intact fuel tank blew up.
Before being forced to retreat, firefighters doused 12 of the 22 oil tank fires.
They were using 32,000 litres of water per minute mixed with foam, trying to cover the blaze in a massive "blanket" to deny it oxygen and cool the oil.
It has also emerged today that two vehicles owned by Irish liquid distribution company, Reynolds Logistics, were caught up in the blaze.
Spokesperson for the company Joe Reynolds said: "Reynolds Logistics are currently in the advance stages of bringing together a major accident training exercise with all the emergency services and a number of expert groups.
"All procedures and protocols in relation to the movement of product and driver training are reviewed and assessed on a constant basis.
"We are conscious that given the lightly dangerous liquids our drivers transport that all measures are put in place to ensure safety levels are paramount, in order to avoid any such incident in Ireland," Mr Reynolds added.