Barnack limestone blind arcading in the north aisle of Peterborough Cathedral was so badly damaged by heat when fire swept through the cathedral at the end of November it will have to be replaced.

A 13th century ceiling at the cathedral was being cleaned by Perry Lithgow in partnership with Hugh Harrison, both conservation specialists, when the fire struck. They had just finished phase four of five phases in the £1.5million conservation of the painting.

The early detection of the fire by the verger saved the ceiling from being burnt but it, like the rest of the cathedral, is covered in thick, greasy soot and work will have to begin again.

The fire started mysteriously in a stack of plastic-seated chairs and was only minutes away from catching light to the wooden choir stalls, from where it would have spread rapidly and caused significant structural damage, when firemen started fighting the blaze.

Even so, the cleaning and restoration work as a result of the fire will cost millions of pounds to repair and involve most of the building trades, a cathedral spokesman told NSS. "The entire organ has to be dismantled and cleaned. That alone will cost half a million pounds."

It was only by providence that the fire was detected at an early stage and the alarm raised. In response, 11 fire engines and about 60 fire fighters attended the blaze and were able to extinguish the flames within 25 minutes. However, it took more than two hours to clear the thick black smoke from the building.

Services continued the same day in a small chapel that was hurriedly cleaned. Within 48 hours the main body of the cathedral had been cleaned sufficiently for services to be resumed.

The real cleaning and restoration work of the stone and woodwork of the fabric of the building will begin in the New Year and is expected to take two years to complete.

The work will be carried out in phases so that the cathedral can remain open throughout and the areas being worked on cordoned off.

The cathedral spokesman said areas of Barnack stone nearest the fire were severely damaged by heat and some of the stone would need replacing. "Some of the pillars are already decaying," he said. It would all need cleaning.

The cathedral dates back to the 12th century and is a Listed Monument. It is about 150m long, 60m wide at the transept and has a tower more than 40m high.