An association representing about 140 former MPs intends to fight any move to strip them of their travel discounts.
It says removing the discounts would breach a legal contract between them and the Government.
The travel allowance entitles former MPs to up to 90 per cent in taxpayer subsidies on their international travel costs.
The president of the Association of Former Members of Parliament, Graham Kelly - a Labour MP from 1987 until 2003 - said the group would resist any efforts to end the subsidies.
Mr Kelly said the allowance, which ended in 1999, was given to MPs as part of legally binding terms of employment and was taken into account when base salary rates were set.
"Our conditions were set at the time for those members - there are not that many involved - and it's a contract between the Government and them which would be broken if it was changed without mutual agreement."
In 2007, the association consulted an employment lawyer on how to protect the perk after businessman John Goulter's independent report on MPs' allowances recommended it be reviewed.
The lawyer stopped work when it became clear the Speaker would not pursue the recommendation.
The issue of the allowance arose again after the release of MPs' travel expenses and discovery that Taito Phillip Field was entitled to the perk, despite being convicted on bribery and corruption charges.
The Green Party wants it reviewed, and Prime Minister John Key and Labour leader Phil Goff have said they would not block this.
But there has been reluctance to remove the perk on a wider scale.
Present and former MPs elected before 1999 are entitled to discounts of 60 per cent on private international travel after nine years of service, 75 per cent after 12 years and 90 per cent after 15 years.
Former MPs' spending is capped at about $10,000 a year, but sitting MPs who are eligible face no limit.
The cost to taxpayers of former MPs' domestic and international travel costs came to $1.1 million in 2005/06 - the only year for which a figure can be found.
A former association secretary and National cabinet member, Dr Ian Shearer, said the ex-MPs had good reasons for keeping the perks.
Dr Shearer, 67, was an MP between 1975 and 1984. He said MPs were far less supported then, and had to make do without electorate office secretarial support, work which often fell to MPs' wives who received no acknowledgement for it.
"The wife invariably ended up doing all of the secretarial work; your home was the office for your political activities. There were a lot less of the fringe benefits available in those days so all of this has to be taken into account in this package.
"That was what the situation was like in those days. The wife and the family were involved in it. It is a heck of a lot easier now."
The travel allowance was introduced in the early 1970s, during the time of Labour Prime Minister Norman Kirk, in lieu of a pay rise for MPs.
Former MP Winston Peters said he didn't want to be interviewed on the subject.
"I'm not going to debase myself. Go and ask someone else."
Former National Party minister Peter Gresham - the 2007 Association president who sought legal advice after Mr Goulter's report - said matters were different for current MPs, but he could see no reason for axing the allowance.
"This was something that ruled at the time and was pertinent to the issues of the time and pertinent to the decisions made by the Higher Salaries Commission when our salaries were being fixed then.
"Clearly there's a big difference between what people used to be paid and what they are paid now."
* How much has it cost taxpayers?
In 2005/06 - $1.1 million for domestic and international travel by former MPs and their spouses.
In 2005/06 - $1.9 million for international travel by sitting MPs and their spouses, as well as spouses' domestic travel.
80 current MPs can get the discount for personal international travel. Of those, 28 will keep it when they leave Parliament. MPs who entered Parliament after 1999 do not get the discounts.