The reform package is part of the response to the MPs' expenses crisis
Emergency plans to reform Parliament in the wake of the expenses scandal have been criticised by peers.
MPs voted through the Parliamentary Standards Bill in three days after two contentious clauses were dropped.
But a Lords committee said it had been drawn up "behind closed doors", seemed most concerned with "public perception" and had been rushed through.
The government says it is "imperative" that a new regulatory body for MPs' expenses be set up quickly.
The Parliamentary Standards Bill cleared its Commons stages last week but only after the government was defeated on a clause that could have allowed things said in debates to be used in court against errant MPs.
Justice Secretary Jack Straw also agreed to drop plans for a statutory code of conduct for all aspects of MPs' behaviour in the face of opposition.
The bill now goes to the House of Lords for scrutiny, and the government wants it to become law by 21 July, when MPs leave for the summer recess.
However, it has been criticised in a report by the Lords' cross-party Constitution Committee, which said "excessively speedy policy making" meant it had not been properly scrutinised.
Even with the two contentious clauses dropped, the committee said the bill raised "serious constitutional repercussions" about the relationship of Parliament and the courts.
They say setting up an independent external regulator for Parliament breaks the convention that it regulates its own affairs.
There appeared to have been no public consultation; instead there had been discussions "conducted under the Chatham House Rules" with other party leaders, the report said.
"It is ironic that provisions designed to restore public confidence in aspects of the operation of Parliament have emerged from behind closed doors," the committee said.
"This is no way in which to legislate on matters which raise complex constitutional and legal issues."
The report said there was "no justification" for abandoning the usual period of consultation, which would have allowed expert views to be heard.
And there was "little sign" policy makers had looked at what worked well and what worked badly in other Parliamentary systems, it said.
Lords leader Baroness Royall told the committee the bill had been "fast tracked" because there was "an urgent public demand to see something done" about regulating MPs' expenses.
But the committee said: "It appears that the key driver for the Bill is public perception, rather than any specific policy outcome."
It added: "The way policy making has been rushed, the lack of public consultation and the limited opportunities given to Parliament to scrutinise the bill all, in our view, fail to meet the minimum requirements of constitutional acceptability."
It also described Gordon Brown's expenses announcement on YouTube as "constitutionally unorthodox".
Several MPs complained during the Commons stages about the way the bill was being rushed through but the government said the main themes had been agreed by all party leaders.
Responding to the report, a Ministry of Justice spokesman said putting accountability and transparency at the heart of Parliament was vital to restore the public's trust and the government had to "act now".
He added: "Such is the depth of public concern about the expenses crisis, and such is the fundamental nature of the failings uncovered, that we must act and act speedily - not with haste and not without consideration, but through the kind of detailed process in which we have engaged over the last few weeks and which will now move to the House of Lords.
"We regard it as imperative to have on the statute book by the end of July a workable scheme to establish an Independent Parliamentary Standards Authority."