Protesters in Amritsar demanded the return of the armour and painting
The Sotheby's auction house says it has no evidence to support claims that body armour in its catalogue once belonged to Guru Gobind Singh.
Guru Gobind Singh was the tenth and last religious head of the Sikhs.
Sikhs in India say the auction of the body armour in London should not go ahead. They want the armour housed in a museum at Amritsar's Golden Temple.
Sotheby's say their catalogue may not have been clear enough on where the armour came from.
The body armour is due to be auctioned on 9 April.
"Sotheby's has researched the provenance of this piece which is believed to date to the 18th century," a statement from the London auction house said.
"In the course of this research, Sotheby's has not found or been given any evidence to indicate ownership of this piece by Guru Gobind Singh.
"We therefore do not deem the piece to be a relic of the Guru.
"We regret if our catalogue notes might not have been sufficiently clear on this point. Should Sotheby's receive any information that provides evidence of ownership, we will of course consider it with the utmost seriousness."
'Not augur well'
The protests have been mainly in the eastern Indian city of Patna - the birthplace of the Guru - and in Amritsar.
Prime Minister Manmohan Singh has been urged to try to stop the auction and help get the armour
"This auction does not augur well," said Gyaan Iqbal Singh, the Chief Priest of Patna Sahib Gurudwara.
"So far," he said, "they have preserved our things and should try and return them at the earliest.
"We would even ask the Indian government and Prime Minister Manmohan Singh to try and get back the armour, as it is his duty as a Sikh as well as Prime Minister of the country."
Guru Gobind Singh was a military commander, as well as a poet and spiritual leader.
He fought many battles with the Mughal emperors and their allies.