In 1978, India knew Pak was 2-3 yrs away from nuclear bomb
Both the US and Britain collectively decided not to inform India about the operation, fearing it would have adverse impact in Pakistan if was leaked from New Delhi.
"Having heard reports that an evolving plan to curb the Pakistani nuclear programme might involve informing the Indian government about US concerns, (US) Ambassador (to Pakistan, Arthur) Hummel suggested that going through with that, especially providing factual information, would have an adverse impact in Pakistan if word of it leaked from New Delhi," said the US Embassy cable issued from New Delhi on November 17, 1978.
Hummel was not worried about leaks from the Nuclear Suppliers Group but "for USG to be seen by Pakistan to be colluding with India in this is [a] totally different matter", said the latest declassified State Department cables.
In response, the State Department said that its officials had instructions not to share US "concerns" with the Indians while the then US Ambassador to India Robert Francis Goheen observed that the Indian government "has known for some time that we believe Pakistan to be bent on achieving a nuclear weapons capability."
While he believed that the embassy had offered "no factual information," he said that Indian diplomat V Shankar had claimed that Pakistan was "two or three years" away from a nuclear capability.