On the eve of a top US official’s visit to New Delhi to wrap up a civilian nuclear agreement, non-proliferation advocates are urging American lawmakers to reject the deal if it is not “consistent” with the law.

In a letter to members of Congress, the group asked them to communicate to the White House that they would oppose any proposed agreement for nuclear cooperation with India that does not explicitly meet all requirements outlined in the US law and other well-established US policies for civil nuclear cooperation.

Undersecretary of the state R. Nicholas Burns is expected to travel to New Delhi on May 21 to seal the nuclear deal. His visit follows meetings in Washington earlier this month with foreign secretary Shiv Shankar Menon, following which both the sides reported significant progress.

The letter has been signed by critics of the US-India civilian nuclear deal, including Hal Bengelsdorf, former director for the office for Non-proliferation Policy at the Energy Department, ambassador George Bunn, a negotiator of the nuclear Non-Proliferation Treaty, Joseph Cirincione at the Centre for American Progress, ambassador Ralph Earle II, former director of the US Arms Control and Disarmament Agency and Daryl G. Kimball, executive director of the Arms Control Association.

The group wants the deal to be terminated if India tests a nuclear weapon. “New Delhi must recognise that other states are under no legal or political obligation to assist India if it defies the world with another nuclear blast,” the letter said.