Essex proved themselves as serious Specsavers County Championship title contenders as they condemned table-toppers Somerset to their first defeat of the season.

Fast bowler Jamie Porter followed up his 5-51 in the first innings by blasting through Somerset's batting line-up with analysis of 4-22 - as the early pacesetters were bowled out for 117 to lose by 151 runs at Chelmsford.

Somerset had arrived at Chelmsford with a 30-point advantage at the summit of Division One, as their quest for their maiden Championship title appeared unassailable.

But they departed with Essex only 13 points adrift, breathing down their necks and looking ominous at home - with the county recording a quartet of hefty victories against Kent, Nottinghamshire, Hampshire and now Somerset at the Cloudfm County Ground.

Somerset were given 269 runs to win, having briskly taken the last four Essex wickets, but were bowled out inside 33 overs.

Porter pounced with the second ball of each of his first two overs - as opening pair Tom Abell and Azhar Ali failed to deal with his pace.

Captain Abell drilled straight back at Porter before Pakistan star Ali edged to Tom Westley at third slip.

James Hildreth and Tom Banton attempted to counter-attack in the hope of ticking down the runs quicker, with the latter hooking a six off Aaron Beard.

The introduction of spinner Simon Harmer, as often happens, brought about the end of a 31-stand with the last delivery of his primary over - as Banton flicked to a stooping Nick Browne at short mid-wicket.

Hildreth, who survived a tough drop from Ryan ten Doeschate at short leg on 25, departed on the stroke of lunch when his attempted upper-cut flicked behind to Adam Wheater.

After the interval, Somerset collapsed, with Porter and Harmer sharing three wickets in 17 balls.

Steven Davies was caught on the crease as he edged the spinner to Sir Alastair Cook at first slip, George Bartlett was well caught by Dan Lawrence at gully before Jamie Overton mimicked his first innings dismissal by picking out Browne on the square-leg boundary.

In form Lewis Gregory provided a punchy antidote to the slide with four maximums off Harmer, including three in a row, all over various degrees of mid-wicket.

But having reached 40 from 35 balls, Beard snapped both Gregory's middle stump and a bail with the second ball of his return to the attack.

Beard wrapped up the victory as Tim Groenewald picked out Browne at deep square-leg and splayed Jack Brooks' stumps.

Earlier, Essex's second innings was wrapped up in 9.5 morning overs as Tim Groenewald claimed his 16th five-wicket haul in first-class cricket.

Fast bowler Groenewald, who was only playing due to Craig Overton's groin injury, followed up his three wickets the previous evening by collapsing Harmer's off-stump with his second ball of the morning.

He thought he had Peter Siddle latter in the same over, as the Australian edged to Jamie Overton at second slip - but the two umpires convened to agree the ball hadn't carried, much to the ire of Somerset.

They needn't have worried as Gregory had him and Adam Wheater caught in the slips soon after before Groenewald wrapped up the innings when Porter nicked to second slip.