A Tory MP Mr Andrew Tyrie called a news conference to release copies of five letters between Mr Blair, his advisers and Mr Gopichand and Mr Srichand Hinduja, who face charges in India of kickbacks from Sweden's Bofors company over a 1986 arms sale.
"The prime minister is now engaged in a cover-up," Mr Tyrie charged. He said Mr Blair should have made the letters available to a recent inquiry by Sir Anthony Hammond into how the two Hindujas got their British passports so quickly under Mr Blair's government.
Mr Blair's office said the letters were "wholly unremarkable" and said Sir Anthony had access to whatever papers he thought relevant to his inquiry, which in March found nothing wrong with the granting of British passports to the Hindujas.
The letters do not touch on the matters of British passports for the brothers; instead they focus on issues such as nuclear tests in India and Pakistan in 1998.
One of them, from Mr Blair to Mr Srichand Hinduja, emphasises "there is no question of excluding India from the mainstream of international policy" although it notes "deep international concern at India and Pakistan's nuclear tests".
It is signed "Best wishes, yours ever, Tony".
In India, a government spokesman said that India used the billionaire Hinduja brothers to help smooth relations with Britain after New Delhi triggered an international outcry when it held nuclear tests in May 1998.