It was conceived as a project that would foster dialogue between Muslims and non-Muslims and act as a bridge between Islam and Christianity. But the completion of a controversial multimillion euro mosque and culture complex is under threat after a prolonged row between its German architect and its Turkish developers.

The Cologne Central Mosque, which was due for completion this summer, has been dogged by controversy since its inception in 2006. But it is now at the centre of a row over everything from soaring building costs and the colour of its facade to allegations that Christian symbols have been secretly hidden in its construction.

Architect Paul Böhm and Ditib, a branch of the Turkish government's religious affairs authority, have tentatively agreed to continue with the project but only after the intervention of a retired Cologne mayor. Fritz Schramma said the parties, whom he unambiguously referred to as "combatants", had agreed to complete the project to create a "transparent, open mosque", but only after much intense dialogue. He said he hoped the mosque would "soon be filled with life".

Böhm's domed mosque, which borrows features from the Ottoman architectural style and has a 55m-high minaret and space for 1,200 worshippers, is destined to become Germany's biggest mosque and one of Europe's largest. Its adjoining culture centre will house everything from a library and prayer rooms to a bazaar, a travel agent and a hairdressers.

The architect, whose father was Gottfried Böhm, a famous architect of Catholic churches, won the commission to design the mosque six years ago. But he has since accused Ditib of a smear campaign against him after it produced a list of extra demands and, he says, repeatedly changed the building plans, causing the initial cost to soar by €17m to €34m. He was later banned from the construction site and his contract terminated, demoting him to the role of adviser.

Ditib, which is responsible for 1,000 mosques in Germany, has presented Böhm with a list of 2,000 construction faults, referring to him as "a brilliant artist but a failed construction manager". It is objecting to the sand-coloured facade and accuses Böhm of surreptitiously working Christian symbols into the building, including crucifixes, as well as an X and a P, which together symbolise Christ. Böhm has denied the allegations.

Lale Akgün, a local Social Democrat politician of Turkish origin, has accused Ditib of becoming "increasingly more conservative and closed, in line with the wider political picture in Turkey", saying that the accusations about the Christian symbols had originated in Ankara. Ditib has refused to comment, except to say the project would go ahead with Böhm acting in an advisory capacity only, "in order to follow the completion of the construction process through to the end".

Since its conception, the mosque has caused considerable tension in the city. Among its opponents are those who are against it for aesthetic reasons – including its sprawling size and towering minarets – and others, particularly the anti-immigrant movement Pro Köln and the writer Ralph Giordano, who argue that it is a political symbol that does not belong in the city and would allow Cologne's Muslims to live in an autonomous world rather than encouraging them to integrate.

But Cologne's mayor Jürgen Roters argues the city's reputation would be irreparably damaged if the mosque, whose outer shell is more or less finished, was not completed.

"It's important that our fellow Muslim citizens get their own dignified place of prayer and meeting, and that the project is not hampered by further rows or misused for anti-foreigner propaganda," he said.

But Böhm, who has complained of unpaid bills and a concerted attempt to push him off the project, said it amounted to a "very delicate and fragile plant" whose completion was far from safe. "The differences of opinion have remained," he said.