Reports that suspected Iranian terrorists have been photographing synagogues in the borough have fuelled fears that an attack is being prepared on a Jewish target.

Last week's Sunday Telegraph claimed that 20 Iranians, many of them students, were involved in surveillance operations on numerous buildings used by the Jewish community in London, and two have subsequently been deported. The article quoted unnamed intelligence sources speculating on a possible link to an al-Qaeda terror attack within the next few months.

Since then, the Sternberg Centre in East End Road, Finchley, which houses the New North London Synagogue (NNLS), has confirmed it was photographed in suspicious circumstances.

Brian Berelowitz, chairman of NNLS, said: "There was a photograph incident some time ago, about three months. The person was stopped. [When I heard of the Sunday Telegraph report] it worried me with the past incident."

He refused to be drawn further on details of the incident, including on whether there was a link to Iran or al-Qaeda.

The Community Security Trust (CST), which provides security measures for most of the UK's Jewish public sites, confirmed that at least one Jewish community building had been targeted, but refused to say which one or if other similar incidents had taken place.

A spokeswoman said: "We have been warned of a Jewish community building being photographed. We have a high level of security at the moment. We are asking members of the community to be very vigilant."

Mr Berelowitz said: "We will have to discuss what level of alert the synagogue takes. But we do not take the decision, the CST takes it."

Iran has a history of terrorist activity abroad as the sponsor of the militant Islamic group Hizbullah.

Hadi Soleimanpour, the former Iranian ambassador to Argentina, was arrested in Britain this August in connection with the bombing of a Jewish community centre in Buenos Aires in 1994 which killed 85 people.

Two Palestinian students, Jawad Botmeh and Samar Alami, were convicted of a car-bomb attack on a Jewish charity in Tally Ho Corner, North Finchley, in 1994, which injured four people.

Al-Qaeda leaders have urged terrorist cells to attack synagogues and Jewish sites, and two al-Qaeda-linked attacks last year were directed at Jewish targets in Mombasa, Kenya, and Casablanca, Morocco.

Barnet is home to the UK's largest Jewish community — around 46,000 — as well as an estimated 5,000 Iranians, many of them opposed to the current regime in Tehran.

Massoud Zabeti, from Golders Green, is a prominent member of the Iranian community with links to the Iranian opposition party and believes that these are clear signs of preparation for an attack. He said: "There have been surveillance operation against Iranian dissidents and foreigners for years — this is typical of how the Iranian regime operates. This is the type of thing which the Iranian people have been warning about for years.

"We should be more vigilant with what they are doing. If we are not more vigilant, there might be an attack."

But the Metropolitan Police do not believe an attack is imminent. A spokesman said: "We have no knowledge of that incident. Since 9/11, the level of threat is, and remains, very high. We do not discuss security matters."

If you see anything suspicious, call New Scotland Yard's anti-terrorist hotline on 0800 789321.