Singh, who retired from service in June, appeared to criticise the UPA government for its handling of national security, suggesting that it was being compromised for vote bank politics.
"I share BJP's hardline orientation on issues of national security. BJP and its other associates like RSS never compromise on this. Other parties tend to compromise due to their vote banks," he told the media as he was welcomed into the party by its president Rajnath Singh.
Besides Singh, Sushil Shastri, son of former prime minister Lal Bahadur Shastri, former Petroleum Secretary Raghav Sharan Pandey and retired railways official Dharam Singh joined the party.
"People who have served the country with distinction have decided to do their bit when it is in trouble. Today is just the begining. You will see many more such people joining BJP in days to come," Rajnath Singh said.
R K Singh, who was being wooed by Bihar Chief Minister Nitish Kumar to join JD(U), and Pandey are from Bihar and may be fielded by BJP from there in Lok Sabha elections, sources said.