MPs have called on Twitter and Facebook to take action after many users were confronted with autoplaying videos of the murder of a US TV news crew.
The footage was suspected to have been posted by the murderer on Wednesday.
Because the sites have set video to play automatically by default, many people saw the video without choosing to when it was shared into their feeds.
A parliamentary group said the firms should ensure that users are warned about graphic content before it plays.
The chair of the cross-party Parliamentary Internet, Communications and Technology Forum (Pictfor) said that both social media sites should automatically sift for such content.
"Facebook, Google, Microsoft and others have already worked together with government and regulators to prevent people being exposed to illegal, extremist content, using both automatic and manual techniques to identify footage.
"Social media, just like traditional media, should consider how shocking other content can be, and make sure consumers are warned appropriately," Matt Warman told the BBC.
The Conservative MP for Boston and Skegness added: "For victims, friends and families it's important to make sure that, in an online world without a watershed, users know what they're about to see and have a reasonable opportunity to stop it."
He said that, while users can change their own settings to stop videos auto-playing, Facebook and Twitter "need to be aware that one size does not fit all".
He said: "Many people who are ordinarily happy that videos play will have seen shocking footage by accident, without warning of its graphic nature."
Why did the videos appear and what was the reaction?
Shortly after the murders of two US journalists as they carried out a live broadcast on Wednesday, it emerged that the suspected gunman had posted his own footage on both Twitter and Facebook.
Those posts were then retweeted and shared by other users. Because video plays automatically on both sites, many people who were connected to someone who had shared the content - and who had not already disabled the default feature - were confronted with it.
Some users began pointing out to those spreading the videos that they were placing them in the feeds of people who may have autoplay activated.
Both sites have rules on what can and cannot be posted and will take down videos that violate them. Facebook, for example, prohibits the use of its network for the commission or celebration of criminal activities.
Twitter took down the account within around 10 minutes of it being posted. That came after a user flagged it to the site. The suspension removed all of the tweets posted by the account, including where its tweets had been retweeted to other users.
The BBC understands that it then manually trawled for other instances of the video and placed graphic content warnings on them. Those warnings would appear to any users who asked to be informed of sensitive content in their settings.
Facebook took a few minutes longer to take down the account. Its spokesman said: "We have removed a profile and a page for violating our Community Standards."
Why do videos play automatically?
The sites say videos that play automatically are easier to use. Moreover, they increase the amount of video being watched and engaged with on their platforms, which makes them more attractive to advertisers.
Facebook started testing the feature in September 2013 and rolled it out three months later. It immediately saw an increase in the amount of video watched on its platforms. In January 2015, it said: "In just one year, the number of video posts per person has increased 75% globally."
Twitter announced it would start using the feature in June 2015, saying it wanted to make videos easier to use on its platforms. "It used to be that watching a video on Twitter required several taps. So when something was unfolding in real time... that extra effort meant you could miss something that you care about."
Can it be turned off?
Yes. While both sites have it turned on by default, it is possible to deactivate it.
On the desktop view, Facebook users must go to their settings, then click "Videos" - the bottom option on the list on the left of the screen - and change the autoplay video option to "off" in the dropdown menu.
On iOS devices, users who want to disable the feature must press the three line icon, then select "Settings" and "Account Settings", before tapping on "Videos and Photos". Then they must tap on "Auto-Play" and "Never Play Videos Automatically".
Android users should begin by pressing the three line icon, then tap "App Settings", before selecting "Video Auto-play" and then turning it off.
Twiter users on desktop must go to their settings and untick the "Video autoplay" option under the "Account" tab.
On iOS, users must take the same route to their settings, then enter then video autoplay options and select "Never play videos automatically". The method is similar on Android devices.
What is going to change?
Neither site has said much publicly about their respective approaches to Wednesday's issues or on their plans to deal with future cases. It is clearly not the first instance in which graphic videos of violent crimes spread quickly on the sites.
In January this year, video footage of the murder of a French policeman during the attacks on the Charlie Hebdo offices in Paris appeared on social media.
Two months later, Facebook introduced new rules and measures - among them, the ability for Facebook staff to add "interstitial" warnings about graphic content to videos, although users could not do so themselves.
The measures were welcomed by the Family Online Safety Institute (Fosi), one of five independent organisations that make up its safety advisory board. But Fosi also said that the site had not gone far enough in protecting users.