Crawley take the points with McLeod's late penalty.
Izale McLeod's spot kick proved to be the difference though, leaving the U's six points from safety with ten points to go.
Having taken an early lead, the U's were behind at half time, before drawing themselves level.
Colchester United's hopes of escaping the bottom four received a blow on Saturday, as a late Crawley Town penalty saw the visitors take all three points.
Minutes after Crawley’s chance, it was the home side that had the break though. Briggs found some space out on the left and he crossed the ball in which was poorly cleared by Lewis Young and it fell to Szmodics on the edge of the area, who hit an unstoppable drive on the half volley past Price and into the bottom corner.
Crawley hit back and hit back fast after Lewis Young worked his way into the box and found Dickson at the front post and he chipped the ball over Walker to level the score.
Both sides certainly had moral after an explosive start and Colchester were almost in front again after Murphy fizzed a low ball across the six yard box but Szmodics failed to get on the end of it as the away fans blew a sigh of relief.
Anthony Wordsworth almost struck a contender for goal of the season after he hit the bar with a thunderous volley inside the area and left Walker flailing.
It was a 'what you do, we can do better' game as, seconds after Wordsworth’s attempt, Szmodics broke away and struck a 25 yard effort which tipped over the bar by Price.
Both sides began to be cautious and a play a more and organised game as the passing and probing continued but there was very little chances towards the end of the first half.
However, minutes before half time, it was a poor defensive moment from Colchester which saw them go a goal down. This time it was Tomlin with the goal as Young found him with a great ball and he curled his effort into the bottom corner.
The home side got off to an excellent start in the second half as their early pressure resulted in a free kick on the edge of the box after Lee Fowler brought down Alex Gilbey. Fox stepped up and curled a sensational free kick round the wall and into the bottom right hand corner to level the score.
Tomlin almost hit back immediately after his header went just wide of the post after he was allowed the time and space to threaten Walker.
Dickson thought he had put the visitors back in front on 59 minutes after he calmly slotted the ball past Walker but his effort was ruled offside.
Moncur had found chances limited since coming on at the interval but his persistence paid off when the ball fell kindly for him on the edge of the box but he only managed to strike the ball straight at the keeper as Price gathered well.
Moncur had another great chance minutes later after great play by Murphy down the right but Moncur couldn’t get his shot on target as it flew over the crossbar.
Crawley felt Walker should have been sent off after he brought down McLeod in the box but the referee gave the keeper only a yellow and issued a penalty which McLeod converted brilliantly to send Walker the wrong way and give his side the lead with less than 15 minutes to play.
The U’s pushed for the equaliser in stoppage time and the comeback was almost complete when the ball found its way to Healey during a scramble from the corner but he fired his overhead kick straight at Price as the game ended in dramatic fashion.