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Romford fell to another disappointing defeat and were left nine points adrift at the bottom of the table on Tuesday.
Having battled to a goalless draw against Grays at the weekend, Boro were back at Aveley's Parkside Stadium for a home fixture which had been switched from Brentwood Arena.
But despite having a lot of the play in the first half they struggled to create many clear cut chances and fell behind on the stroke of half time.
The visitors got their noses in front when the ball was played into the box and an unmarked Daniel Smith headed home from close range and Boro were reduced to 10 men soon after the restart when Tambeson Eyong was dismissed for a second bookable offence.
Witham doubled their advantage moments later through Jack West and went 3-0 up when Scott Kemp netted.
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Isaac Nkosi added a fourth almost immediately, before Adam Morgan hit back for the home side.
Ejiro Okosieme struck in stoppage time but it was scant consolation for Boro, who saw fellow strugglers Felixstowe & Walton pick up a point from a 2-2 draw with Coggeshall Town.
Glenn Tamplin's men have six games in hand on their relegation rivals, who they entertain in a real six-pointer on Saturday.
Boro won 2-1 when the sides met on the Suffolk coast in early December, thanks to goals from Dominic McHale and Morgan, but have taken just one point from a possible 15 since that day.
Romford: Cafer, Eyong, Bacon, Joseph-Baker, Bonnett-Johnson, Ekosieme, Toussaint, Morgan (Oulakolu 82), McLeod, Theophanous, Sprague. Unused subs: Adelowo, Aziaya, Rochester, Exley Banks.