"It is not clear to us what exactly is the implication of your statement that the boundaries of Sikkim and Bhutan do not fall within the scope of the present discussion," Mr Nehru wrote in a letter to his then Chinese counterpart Zhou Enlai on September 26, 1959.
"In fact, Chinese maps show sizable areas of Bhutan as part of Tibet," Mr Nehru said in the letter accessed by PTI here.
In the lengthy letter highlighting India's stand on the boundary dispute, Mr Nehru wrote that under treaty relationships with Bhutan, the Government of India is the only competent authority to take up with other Governments matters concerning Bhutan's external relations, and in fact it has taken up with China a number of matters on behalf of the Bhutan Government.
"The rectification of errors in Chinese maps regarding the boundary of Bhutan with Tibet is therefore a matter which has to be discussed along with the boundary of India with the Tibet region of China in the same sector," he wrote.
After asserting that Chinese maps are showing sizable areas of Bhutan as part of China, Mr Nehru referred to the 1890 Sino-British treaty granting India's sovereignty to Sikkim.
"As regards Sikkim, the Chinese Government recognised as far back as 1890 that the Government of India 'has direct and exclusive control over the internal administration and foreign relations of that State'. This Convention of 1890 also defined the boundary between Sikkim and Tibet; and the boundary was later, in 1895, demarcated. There is thus no dispute regarding the boundary of Sikkim with the Tibet region," Mr Nehru said.
China and India have been engaged in a standoff in the Doka La area near the Bhutan trijunction for almost a month.
Sikkim, which became a part of India in May 1976, is the only state which has a demarcated border with China. The lines are based on a treaty signed with the Chinese in 1898. Doka La is the Indian name for the region which Bhutan recognises as Dokalam, while China claims it as part of its Donglang region.
Chinese are making attempts to build a road which was objected to by Bhutan.
The Ambassador of Bhutan has a lodged a protest with the Chinese Government through their Embassy in New Delhi on June 20. Bhutan has no diplomatic relations with China.