The victim, who is from the Clacton area, has not yet been named by police.
However, tributes began flooding onto the social networking site Twitter using the hashtag #RIPJay.
One Twitter user, under the name Mary-Ellen1, said:
RIP Jay my little brother, taken too young. We will get you justice.
RIPJay, the mark you left on this Earth is irreplaceable, the mark you left in our hearts is unforgettable.
More top news
In a wide-ranging Peston on Sunday interview, he said Parliament security needs reviewing and the PM's counter-terror strategy is flawed.
The terror attack carried out by Khalid Masood at Westminster that left four people dead lasted less than two minutes say police.
Two knives were found at the scene in Islington, North London, and the suspects fled on foot before being arrested.