Since emerging from the oven in the year of Napoleon Bonaparte's death, the bun has been carefully preserved by the maternal family of Nancy Titman.

The currants have gone and the bun is brittle but Mrs Titman says the indent of the cross can still be seen – after being kept in a cardboard box for 186 years.

Mrs Titman, who lives in Church Street, said the bun was kept because of the supposed medicinal properties and longevity of goods baked on that holiest of days.

She said: "My mother used to say that they never thought it would go mouldy. They were times of faith."

Mrs Titman, who taught at Deeping St Nicholas Primary School for 21 years, said hot cross buns baked on Good Friday are kept as they help ease indigestion, colic and other complaints.

But Pinchbeck baker Conrad Taylor said the bun must have been treated as the normal shelf life is three days.

He said: "They must have treated it with some sort of varnish or preservative to keep the mould out.

Spalding's Canon Tim Barker said he has read stories of people shaving fossilised hot cross buns on to wounds to help them heal.

He said: "It is amazing it has lasted that long. Hot cross buns don't last much beyond Good Friday in our house. They get eaten!"