Labour has already made it clear that it will not support a little-noticed clause that has been added to an otherwise largely uncontroversial Defamation Bill to bring about the fundamental constitutional change.
The proposal in the Bill would allow an MP to waive the centuries-old Parliamentary privilege under which freedom of speech in Parliament cannot be questioned in a court. But MPs would still not be able to be sued over what they say in Parliament about outsiders.
The attempt to alter Article 9 of the Bill of Rights, which conferred the immunity, came after the High Court stopped Mr Hamilton, the former corporate affairs minister, and the lobbying company Ian Greer Associates, from suing the Guardian. The newspaper's lawyers, Lovell White Durrant, successfully argued that Article 9 would prevent it from properly cross examining Mr Hamilton over alleged payments from Mohamed al-Fayed, the chairman of Harrods, and allegations that he failed to declare an expenses- paid stay in the Ritz hotel in Paris, owned by Mr Fayed.
MPs are technically being given a free vote tonight but Tory backbenchers will be left in no doubt of the Government's strong support for amendment in a secret whipping operation.
The amendment from the recently appointed Law Lord, Lord Hoffman, was passed following two failed attempts, after Government business managers packed the chamber with sympathetic Tory peers, including Margaret Thatcher. But Lord Hoffman was away from the division lobby at the crucial time and ended up not voting for his own amendment.
Mr Hamilton has argued that MPs are "uniquely hobbled" by Article 9, but even some Tory MPs have privately voiced fears that the new right to sue over reports of their Parliament-related activities is too extensive, too weighted towards Parliamentarians and could produce a host of unintended consequences.