Adam Musikant, co-ordinator of the appeal, said he was delighted to secure royal support for “a funding appeal of national importance.
“Bevis Marks has not just been a focal point for Jewish life in the UK, but for the wider communities with which it has interacted. We look forward to developing a religious, cultural and educational centre to tell its unique story.”
Apart from its historic Jewish significance, the Grade 1-listed building is a symbol of integration, believed to be the country’s oldest functioning non-Christian place of worship.
Rony Sabah, chairman of the development committee, said from the inception, it was "never a project of a museum. It is a living synagogue."
They hope to begin work on the site next autumn. The new exhibition area will utilise the courtyard and the surrounding space, as well as the undercroft below the sanctuary.
Some of its historic artefacts will be on display in one venue for the first time, while oral histories and digital archives will help to put the story of the synagogue in a wider London context. A partnership has been announced with the Jewish Museum, where Prince Charles is already patron, to run school visits.
When he attended the synagogue’s 300th anniversary service in 2001, Rabbi Levy reminded him that it was the last King Charles, Charles II, who had confirmed the legal status of Jews in the City in 1664 - as long as they conducted themselves “peaceably and quietly” and “without scandal” to the government.
It is understood that Rabbi Levy approached his former pupil, the historian Simon Sebag-Montefiore, who is a friend of the prince, about whether the heir to the throne might be interested.
The construction of the building may have been the occasion of one of the country’s first acts of interfaith benevolence.
The architect Joseph Avis, who is thought to have been a Quaker, reputedly refused to make any profit and returned any money once his costs had been met.
According to tradition, there is even an early royal link to the building. Queen Anne is said to have donated an oak beam from a Royal Navy ship for the synagogue’s roof.
Providing regular services for more than three centuries, the synagogue is believed by some to be the oldest in continuous use anywhere in the Jewish world. (Prague's medieval Altneu Synagogue, for example, was closed during the Nazi occupation).
While Bevis Marks escaped untouched in the Blitz, it was damaged in the 1990s by IRA bombs.
Shalom Morris, its rabbi, said the development project would be "an opportunity for us to educate the broader Jewish community and British society about the the heritage of Bevis Marks and hopefully have a positive impact on the values of our community".
It represented the story of those who were committed to Judaism but sought to integrate into Britain, he said, "and that is the legacy of British Jewry. This is what we want to continue to promote and celebrate."
The appeal has already attracted support from two charitable trusts, the Exilarch Foundation, run by the Dangoor family, and the Joir & Kato Weisz Foundation, established by George Weisz.
A spokesman for Mr Weisz said, "George loves the services and the beautiful music at Bevis Marks. His cousin, Jeremy Schonfield, regularly leads services there.
"The synagogue has one of the finest collections of antique silver, which is sitting in a vault in the City and hasn't been seen in living memory. When we heard the synagogue wanted to do something, we came on board as George is passionate about art."
