Pressure is growing on David Cameron to identify the mystery Tory MP who deliberately scuppered a landmark anti-poverty bill that could have stopped "vulture" bankers profiteering from the developing world's debt burdens.

Debt campaigners have reacted in fury and disbelief to the killing of the bill and Labour MP Sally Keeble, one of the bill's backers, has accused the Conservatives of "duplicity" by pretending to back the legislation and then sabotaging it at the last minute.

Campaigners are now calling on the leader of the opposition to clarify his view of the bill and asking whether the MP concerned will be identified. The international development secretary, Douglas Alexander, has sent a letter to Cameron demanding an explanation.

The frustration has been compounded by the secrecy surrounding the events in the Commons last night. During the reading, three Tory MPs were seen to huddle together on the benches before one shouted the word "object!", which under parliamentary procedure effectively stopped the bill passing.

Three Conservatives were in the chamber – Christopher Chope, Andrew Robathan and Simon Burns – but none has admitted intervening. The Tory treasury spokesman, David Gauke, who was on the committee that debated the bill, insisted the Conservatives had wanted to see it go through and that the MPs, two of whom are Tory whips, did not have the support of the frontbench. He said he did not know which one had made the objection. "We have our suspicions," he added. "It is a pity. Our view was let's go with the bill but that was not to be. Everyone recognises that this was a rushed process."

But Keeble said that there had been plenty of time to debate the bill, both for two hours in the chamber and at committee stage. "All concerns that had been raised had been dealt with and the bill had been watered down already as a compromise to the Conservatives," she said.

"It's blatantly obvious that this was duplicitous behaviour by the Conservatives whose commitment to international development is deeply suspect. The three men went into a huddle and then no one can see who actually objects. It's disgraceful behaviour."

Nick Dearden, director of Jubilee Debt Campaign, said: "It is an outrage that one MP has taken it upon himself to effectively kill a bill which has the support of the vast majority of the house. His move will mean many of the poorest countries in the world will continue suffering at the hands of reckless and unethical investors.

"This action has destroyed the hopes of many people across the developing world that we might put an end to the appalling practice of vulture funds."

Vulture funds buy up the debts of poor countries, often at a fraction of their face value, and pursue them through the international courts, in many instances despite agreements by other creditors to give the country debt relief.

Campaigners wanted the legislation to apply retrospectively, because it could help countries such as Liberia, which lost a £13m case in London against two vulture funds late last year. The Liberian president, Ellen Johnson Sirleaf, has urged parliament to pass the new law. The scuppering came a day after the former Tanzanian president, Benjamin William Mkapa, backed the bill, saying: "I hope the international community joins hands to put an end to these deplorable activities of the vulture funds." The bill also has the support of Archbishop Desmond Tutu.

Andrew Gwynne the Labour MP who proposed the bill, said: "It is staggering the Conservatives are still unwilling to support even the most basic legislation."