Synagogue members have been contacted by canvassers who claimed a Conservative Party candidate was endorsed by the congregation.
Members of South Hampstead Synagogue were called on Tuesday evening by people claiming to be ringing on behalf of the Conservative Party.
The calls raised concerns that the synagogue's membership list may have been leaked or stolen.
SH chairman Michael Hatchwell was forced to email congregants alerting them to the canvassing. But he said the membership list had not been released.
One member, who did not want to be named, said: "The caller was well-spoken, she said she was calling from South Hampstead. I asked 'South Hampstead what?' and she said the synagogue.
"She started going on about a group of women from the Conservative Party, implying the synagogue was endorsing the party.
"At that point I asked who she was. She said she was called Caroline and said I knew her and I would recognise her from a kiddush. But I haven't been to the shul for maybe over a year, so that made alarm bells ring for me."
The caller hung up after being asked to identify herself further.
But Mr Hatchwell said the synagogue did not endorse any candidate. He said: "They have not got the details from the shul. It's nothing to do with us. I have no idea who the callers were. I believe they all say different things.
"I've checked with everyone who has access to the list and no one has passed it on, nor would we. These calls were not sanctioned or approved by us."
He said it was possible a canvasser could use an electoral roll and pick out names of people they recognised.
Chris Philp, Tory candidate for Hampstead and Kilburn, said: "I know for a fact that no-one on my campaign team had a list of synagogue members. The woman caller is not a member of my team.
"I know we had a lot of supporters in the synagogue keen to spread the word, but we certainly did not call in this way."