The Indian government is likely to buy a large archive related to Mahatma Gandhi's time in South Africa, including many letters throwing light on his controversial relationship with architect Hermann Kallenbach, during an auction in London on July 10.

According to auctioneers Sotheby's, the archive comprising letters, documents and photographs is expected to be auctioned for between 500,000 and 700,000 pounds.

An unnamed highly placed Indian official told the BBC that the Indian government "was close to securing a deal" with Sotheby's after a report by experts who examined the archive in London "strongly advised" the government to bid for it.

Sotheby's did not respond to the likelihood of India buying the archive, but said: "The Gandhi-Kallenbach Archive is scheduled for auction at Sotheby's in London on the 10th July 2012."

It also did not respond to reports that Indian experts had examined the archive.

One of the experts who examined the archive is reported by the BBC as saying that it is "very well preserved and of inestimable value".

The auctioneer's Catalogue Note on the archive says that it "is richly informative of the important (and occasionally misunderstood) friendship between the two men, and is a key biographical source for Gandhi".

In one of the letters, Gandhi writes about Kallenbach on 25 March 1945: "...He used to say to me often that when I was deserted by the whole world, I would find him to be a true friend going with me, if need be, to the ends of the earth in search of Truth..."

Kallenbach (1871-1945) met Gandhi in 1904 in South Africa, and the two since then remained in touch.

In 1910, Kallenbach purchased a 1,100 acre farm at Lawley, 20 miles from Johannesburg, which he gave to Gandhi. The two were closely involved in operating the farm.

The archive for auction not only includes documents relating to the purchase but also reveals the two men acquiring and tending fruit trees, ensuring water supply, and arguing with neighbours over grazing rights, Sotheby's said.

The archive is described as including "poignant letters" by the deeply troubled Harilal, Gandhi's first son, and reveals Kallenbach's deep friendship in particular with Gandhi's second son Manilal, who remained living on Phoenix Settlement in South Africa, and his third son Ramdas.

"These letters, together with those by other family members, Mahadev Desai, and other of Gandhi's close associates in India, provide a detailed portrait of Gandhi's personal life in India", the Catalogue Note says.

"This extraordinarily rich archive stands...as a testament to a hugely significant figure in the life of Gandhi and a key member of his inner circle," it added.