England's Hughes won a photo-finish by the slimmest of margins - both were awarded times of 20.12 - and had just completed his lap of honour when he was told that he had been scratched from the race for hitting Richards over the last few metres.
Richards was awarded the gold with Canada's Aaron Brown second, while Northern Ireland's Leon Reid was elevated to bronze, collapsing to the floor in emotion when he heard the news.
"Coming down the home straight, he was ahead, and when I started to catch him, his hand came across and he hit me," said Richards.
"I think I was strong enough down the straight, I was closing on him really fast but when I got hit that knocked me back a bit."
England appealed the disqualification but it was dismissed.
There was no controversy in the women's half-lap race with Bahamian Shaunae Miller-Uibo holding off Jamaica's Shericka Jackson to take the gold in 22.09 seconds, Olympic champion Elaine Thompson trailing in fourth to miss out on a medal.
No-one got close enough to hit Miller-Uibo as, with her distinctive silver hair trailing behind her, the Olympic 400 metre champion secured a victory which encouraged her ambitions to run the 200-400 double at the 2020 Tokyo Olympics.
"The 200 isn't my key event, I'm doing this to get the speed work in and it shows I'm on track for the 400," she said before receiving her gold medal from Usain Bolt.
"The double is going to be a hard feat to accomplish, and I'm ready for it. I've been training for it."
Double Olympic sprint champion Thompson, who made a poor start but still just failed to overhaul England's Dina Asher-Smith for third place, said she was satisfied with her race.
"I'm feeling great," she said. "It's still early in the season. [Shaunae's] getting better and better, she has the strength for the 400 and the speed for the 200."
Janieve Russell did win gold for Jamaica in the women's 400 hurdles while Kenya's wait for a first athletics title of the Games finally ended in the 800m on the fifth day of the programme.
In the absence of world record holder David Rudisha, it fell to Kinyamal to provide it in one minute, 45.11 seconds, even if he almost had the gold medal snatched out of his hands on the line by England's Langford, who ran a personal best for silver.
Kenya's Wycliffe Kinyamal held off surging Kyle Langford to win the 800m title in another thrilling finish, while 400m hurdles prodigy Kyron McMaster gave the British Virgin Islands its first Commonwealth Games gold medal in 48.25 seconds.
Another packed house enjoyed more Australian field success through Kurtis Marschall in the pole vault and Dani Stevens in the women's discus but for drama it was hard to beat the penultimate race of the evening on the track.
