A £15m deal to secure the future of a collection of 17th-century paintings is in doubt after a philanthropist withdrew his offer of financial support.

In March, investment banker Jonathan Ruffer announced plans to purchase 13 works by Francisco Zurbarán from the Church Commissioners, which manages investments on behalf of the Church of England, through a new charitable trust.

The deal would have ensured the collection remained at Auckland Castle, the home of the bishop of Durham, where they have hung since 1756.

At the same time, the Church Commissioners revealed plans to open up the castle and its grounds to provide greater public access. But Ruffer withdrew his offer last month, blaming “insurmountable” conditions that were put on the sale by the Church Commissioners.

In a statement, Ruffer said: “The conditions include the grant of planning consent for a new See House [the plans for which have been abandoned by the Church Commissioners] and the satisfactory grant of planning consent on land that I understand is not even in a development area. I have no option but to withdraw my offer. This is a great sadness and an embarrassment.”

But the businessman added that he would honour his £15m pledge to support the north-east. It is unclear what will now happen to the Zurbaráns. A spokesman for the Church Commissioners said negotiations were continuing.

He added: “We remain committed to helping to provide the best possible solution for Auckland Castle and its  paintings – one that will benefit the region, town, its residents and the church.”

The bishop of Durham, reverend Justin Welby, said: “Everything possible needs to be done to ensure that the paintings remain on the site designed for them, and that Auckland Castle is developed as a major cultural and tourist resource for the area.”