Amnesty International has been accused of sending out an email "filled with hate, lies, distorted facts and misrepresentation of reality" by a relative of one of the 3 Israeli teenagers murdered on the West Bank last month, after its UK crisis campaigns manager appealed to supporters to lobby against arms sales to Israel.

In 2011, Krystian Benedict, in whose name the email was sent, threatened to "smack" a pro-Israel campaigner. The following year he was subject to a disciplinary tribunal after tweeting a joke about three Jewish MPs. The panel found that the joke was "inappropriate and offensive".

Last week Mr Benedict distributed an email which said, in reference to IDF injuries to civilians, that there was "a risk arms from the UK are facilitating such war crimes".

In a letter to Kate Allen, Amnesty's UK Director, Elon Perry said the email had failed to mention Israeli victims. The organisation also ignored Hamas's use of children and civilians "as a shield when firing their rockets from civilian buildings", he said. "Israel is accused of killing civilians because Hamas fires rockets at Israel only from civilian population areas. I have spoken with Palestinian civilians and they have told me that they hate Hamas and are afraid of them. One of them even said to me, 'All our food commodities come from Israel and in return we send rockets'."

In response, Ms Allen said that Amnesty had repeatedly condemned attacks on civilians by both sides. Hamas's use of crude rockets to attack targets in Israel, she said, was "a clear war crime and we condemn it as such, while the horrible murder of Eyal Yifrach and the two other Israeli students was a disgusting act.

"However, we are extremely concerned that Israel's attacks on densely-populated neighbourhoods of Gaza appear to have included indiscriminate and disproportionate bombardments of homes and hospitals".