NEW DELHI, Oct 1 (Reuters) - UK nationals visiting India this month will have to undergo mandatory quarantine even if they are fully vaccinated against COVID-19 in retaliation for UK curbs on Indian citizens, a senior government source in New Delhi said on Friday. The new rules, which take effect from Monday, follow the UK decision to require Indians vaccinated with AstraZeneca"s Covishield to quarantine. read more India mainly relies on the Covishield vaccine, developed by AstraZeneca (AZN.L) and manufactured in India by Pune-based Serum Institute, which is not recognised by Britain under new rules, despite being identical to the doses given to millions of Britons. AstraZeneca is one of the key providers to Britain"s vaccination program, along with U.S. peers Moderna (MRNA.O) and Pfizer (PFE.N). "India has decided to impose reciprocity on UK nationals arriving in India from the UK," the source said, declining to be identified in line with government policy. Government sources in New Delhi had last month warned of retaliation after Foreign Minister Subrahmanyam Jaishankar said he had urged for an early resolution after a meeting with his British counterpart at the United Nations General Assembly in New York.