Charlie Allison struck a maiden half-century in only his third first-team innings to help Essex pull off an extraordinary first victory of the season in the Metro Bank One-Day Cup.

The 18-year-old younger brother of seamer Ben lived up to his promise in the second eleven with a mature knock of 70 that included three sixes and even outshone his captain Tom Westley’s patient 80 from 90 balls.

However, Middlesex looked well on track to record their own first win of the campaign when they reached 236 for 3 with 15 overs left chasing 299 to win.

But despite Mark Stoneman setting the foundations for their reply with a dozen fours and two sixes in a 97-ball 93, Middlesex collapsed to lose by three runs with seven balls left to the delight of a large Chelmsford crowd.

And it was Allison whose throw from the third-man boundary ran out Ethan Bamber as the final Middlesex pair tried to steal the runs to decide the match.

Middlesex had 100 on the board inside the 15 overs of their reply with Stoneman reaching his fifty from just 39 balls. It included two sixes, one scooped off Aaron Beard and another pulled off Jamie Porter. He might have perished there and then had Michael Pepper’s throw been more accurate into the wicketkeeper’s gloves.

He capitalised on the reprieve with an extravagant reverse-sweep for four off Simon Harmer. James Cracknell had been slightly overshadowed in the stand of 124, though he took two successive boundaries off Harmer before he was brilliantly caught in the covers by Robin Das for 48.

The wicket had no significant impact on Middlesex’s progress and they were halfway to their target in 22 overs, though Sam Robson was dropped on 20 at backward point by Das.

Essex thought they had Robson soon after for a catch behind off Beau Webster that left umpire Paul Nicholls unmoved and the bowler showing his frustration by slamming the ball to the ground. Robson was finally removed for 26, driving at Aaron Beard and being bowled.

Stoneman’s stay eight minutes shy of two hours was ended when he swung Beard to Porter on the deep fine-leg boundary.

At that point Middlesex threatened to implode with three wickets going down in 14 balls. Jack Davies was bamboozled by one from Harmer before Ryan Higgins dragged on from Webster and Luke Hollman was run out without facing a ball. It got worse when Josh de Caires was bowled by Porter to leave Middlesex still 42 runs short with 10 overs remaining.

Martin Andersson had helped Simpson take them within nine runs of victory when he was bowled by Webster. He was followed, two runs later, by Simpson, bowled by Westley. And then came the unlikely denouement.

On a pitch that took spin early on, de Caires’s three wickets in eight balls halted an Essex onslaught led by Pepper, who took a liking to Middlesex debutant Isaan Kaushal, hitting him for four fours and a six over extra cover before the 21-year-old seamer was withdrawn from the attack. However, when Kaushel returned late on he claimed his maiden first-team wicket when Will Buttleman picked out deep mid-on.

Pepper swept de Caires’s first ball for his sixth boundary but fell next ball for an 18-ball 31 to a stupendous running catch over his shoulder at deep long leg by Andersson. Das then pulled de Caires invitingly to deep square leg, and two balls later Webster advanced too far and turned in dismay to see John Simpson removing the bails.

Luc Benkenstein helped steady the ship in a 55-run partnership with Westley. But after lofting de Caires for six over long leg, he took a wild swipe at Luke Hollman, who then had Harmer playing all round one to be lbw.

Westley’s innings had few thrills, though he carved Robson through extra cover for his sixth boundary the ball before he was beaten by one that turned from outside off and took middle stump.

Allsion had played calmly alongside Westley but cut loose once his captain had departed. He reached his fifty from 59 balls with a six to long-off in a Robson over that also went for a maximum over long-leg by Buttleman. Allison’s 65-ball innings ended when he played over a ball from Bamber.