Somerset were given a boost when skipper Lewis Gregory declared himself fit to bowl but his two overs proved expensive as Allison and Critchley continued where they had left off the previous evening.

The pair had taken the score from 127-3 to 339 when Leach found the edge of Critchley’s bat and Tom Abell took a juggling catch at second slip.

Encouraged by that success, Leach added potency to his accuracy. It was 346-5 when Allison’s exceptional contribution ended, wicketkeeper James Rew taking a good catch to end a 254-ball innings from the 21-year-old, who had begun the day on 105.

Leach continued to pitch the ball in an area that made maximum use of the last day pitch and was rewarded again with the total on 350 as Michael Pepper edged to Abell, who took his second catch diving to his left.

When Noah Thain, on 11, fell to the same combination of bowler and fielder, Essex were 367-7 and just 56 runs ahead.

Simon Harmer and Shane Snater added 27 up to lunch, but Archie Vaughan joined the party in the first over after the interval when he turned a ball pitched wide and clipped the off stump as Snater played no shot.

Vaughan quickly followed up by ripping another delivery back to defeat Harmer’s defensive shot and bowl him for 24 and then pinned Jamie Porter lbw for figures of 3-123.

The only remaining concern for Somerset was whether Harmer or Critchley could match Leach’s efforts on the worn surface.

Harmer was given the new ball, but Josh Thomas and Tom Lammonby produced an array of attacking shots from the outset to raise a half-century stand off just 6.5 overs.

Thomas (31) fell to a top-edged sweep off Harmer with the total on 59 and Lewis Goldsworthy followed cheaply to the same bowler before Lammonby, who was dropped on 27, hit the winning boundary to finish 39 not out.

Report by ECB Reporters’ Network, supported by Rothesay.

Somerset spin bowler Jack Leach:

“When we turned up this morning, head coach Jason Kerr told us to focus on the fact that Essex were effectively three for three and to take things from there.

“In the end, the team performance is what matters most and we can take a lot of confidence from beating a really good side.

“The Championship race is wide open and there is a lot of excitement in the dressing room because we know what winning a first ever title would mean to us and our supporters. We are right in there with a chance.

“It has not been an easy season for me and I still don’t think I am bowling at my best. But hopefully that will come over the last five games.”

Essex director of cricket Chris Silverwood:

“We have won day three, but lost all the other days, so Somerset fully deserved their victory.

“I stand by our decision to bowl first because of how conditions were at the start of the game. We simply didn’t bowl well enough on day one.

“We have an experienced attack and I wouldn’t offer any excuses for that. On the second morning we bowled a lot better, but by then we were behind in the game.

“If you don’t perform to your best at this level, teams like Somerset will beat you. And we were not at our best in this match.”