Essex 423 (Lawrence 154, ten Doeschate 86, Foster 54, Klein 4-107) beat Leicestershire 238 (Cosgrove 71, Robson 52, Porter 4-50) and 175 (Masters 4-39, Porter 4-49) by an innings and 10 runs

Leicestershire lost their coach and their promotion hopes within a few hours with Andrew McDonald confirming a return to Australia with a year of his contract remaining and Essex inflicting their heaviest defeat of the season.

McDonald, two years into a three-year deal, has accepted the Victoria coaching position, having already been named as the new Melbourne Renegades coach.

McDonald held a meeting with Wasim Khan, the Leicestershire chief executive, on Thursday evening at which he explained the lure of the Victoria job and his family's desire to return to Australia. Leicestershire had seemed resigned to the news for a few days and did not attempt to contest McDonald's departure. Pierre de Bruyn, currently in charge of the seconds, steps into the head coach role, with Graeme Welch, the former Derbyshire coach, lined up as assistant.

"This decision has not been easy for Andrew and we are naturally disappointed to lose a person and coach of his calibre. We fully understand and respect the reasons for his decision," said Khan.

"Andrew has done a fantastic job here in his two years as Elite Performance Director, with the team making great strides forward. Performances and results have drastically improved and Andrew has left a strong platform to build on. We will now look to build on his excellent work.

It is an understandable decision from McDonald. He represented Victoria and Melbourne Renegades as a player and might well be considered a viable candidate the next time the Australia coaching role becomes vacant.

"It has been an extremely difficult decision to leave Leicestershire County Cricket Club but I have to put my family first and we are now going to settle back home in Australia.

"I'd like to thank the club for this fantastic opportunity, I'll always be grateful for having the chance to both play and coach at the Fischer County Ground, and will continue to follow the club's fortunes from afar.

"I also appreciate the fact that Leicestershire have allowed me the opportunity to return home and wish the club every success for the future."

It will sting McDonald to depart in such circumstances. After a season of improvement in Championship cricket, Leicestershire succumbed to defeat by an innings and 10 runs on the third day against Essex. Having had Essex reeling at 68 for 5 in their first innings, it was a remarkable turnaround.

They may reflect, though, that they lost the match as much in their own first innings, with a total of 238 probably around 100 below par; the absence of a spinner may also have cost them in Essex's innings. Leicestershire's spinners have contributed fewer than 100 overs in the Championship all season.

They also fell, in part at least, victim to their own tactics here. Having opted to play this game on a used surface in the knowledge that there would be some uneven bounce as the game progressed, they struggled to deal with the probing length of Jamie Porter and David Masters once the pitch started to deteriorate.

Both men made a few deliveries spit off a length, rapping batsmen on the gloves and body. After Paul Horton and Angus Robson were drawn into feeling for balls outside off, Mark Cosgrove missed a straight one as he shuffled across and tried to work it into the leg side and Mark Pettini edged a brute of a ball that climbed and took his outside edge.

Ned Eckersley clipped one off his hip to Kishen Velani, the substitute on to allow Alastair Cook to attend the England meeting about the Bangladesh tour, at square leg. A terrible mix-up between Ben Raine and Richard Jones, with both men stranded in mid-pitch looking unimpressed with one another, summed up a disappointing day for Leicestershire. They are now 44 points behind Essex with three matches to play.

The fact that they made it towards the end of August with promotion hopes intact represents improvement, although how much progress has been made remains open to debate.

Leicestershire were fifth in Group A of the Royal London Cup (with three wins) and eighth in the North Group of the NatWest Blast (with four wins) in 2014, the year before McDonald arrived. This year they were eighth in the Royal London Cup (with two wins) and ninth in the North Group of The Blast (with four wins). Their improvement in the Championship is welcome, but the lack of progress of home-grown players - a substantial part of the role of any club and a key justification for the level of funding they receive - is a major caveat.

De Bruyn, the new coach, will inherit some good, experienced players and an improved culture of greater expectations. But there remains a lot of work ahead if Leicestershire are to deny the growing band of those who think of them, and several other counties, as a pesky irrelevance and hangover of a bygone age.

Their supporters will, quite rightly, point to their role in the development of several fine players now with other counties. But if the 18-county system is to survive - and it would hard to exaggerate what peril it is in right now - it has to win over the doubters, too.

Essex go into the final three games as promotion favourites but, with a final match against second-placed Kent, can take nothing for granted. They look to have a batting line-up that could serve them well in Division One, though the absence of a top-class spinner is one obvious weakness. The progress since Chris Silverwood took over as coach a year ago - initially on an interim basis - and Ryan ten Doeschate was given the Championship captaincy at the start of this season, is outstanding.

August 26, 10.27am: This article was updated following confirmation of McDonald's departure