LONDON (Reuters) - The number of anti-Semitic incidents in Britain rose by 11 percent in the first six months of the year reflecting a worrying trend of intolerance across the country, a Jewish advisory body said on Thursday.

The Community Security Trust, which advises Britain's estimated 260,000 Jews on security matters, recorded 557 incidents from January until June, the second highest number for the first period of a year since it started collecting figures in 1984.

The year-on-year rise comes at a time when politicians and police have warned about growing levels of hate crimes since Britons voted to leave the European Union on June 23, with Muslims and Eastern Europeans particularly targeted, although the CST said it had not seen a spike after the referendum.

It also follows accusations of anti-Semitism against figures in the opposition Labour Party.

"This rise in reported anti-Semitism comes at a time when division, intolerance and prejudice appear to be deepening within our society," said CST Chief Executive David Delew.

"Reversing this worrying trend requires real leadership from all political parties, and for the social media companies to take their share of the responsibility."

In its report, the CST said the average monthly number of hate incidents directed at the Jewish community was now almost double that recorded in 2011-13.

Social media was the medium for almost a quarter of all incidents and had become an "essential tool" for campaigns of anti-Semitic harassment aimed at Jewish public figures and politicians.

One Labour Jewish lawmaker, Luciana Berger, reported in April she had received thousands of online abusive messages including threats to rape her.

The CST said most of the occurrences took place in April, May and June when anti-Semitism was in the news with Berger's disclosure and Labour launching an inquiry into whether the party had a problem in its ranks ranging from a former London Mayor to Oxford University students.

"The message should go out to everyone that we will not stand for anti-Semitism. Perpetrators of hate crimes against Jews will be caught and prosecuted and the bystanders must be educated," said lawmaker John Mann, chairman of the All-Party Parliamentary Group Against Antisemitism.

(Reporting by Michael Holden; editing by Stephen Addison)