The EU Withdrawal Bill has been branded "constitutionally unacceptable" by the Lords Constitutional Committee ahead of a debate on it in the House of Lords.
The committee noted that the unprecedented challenge of converting existing EU law into UK law is further complicated because "in many areas the final shape of that law will depend on the outcome of the UK's negotiations with the EU".
The report asserted that the bill was "fundamentally flawed from a constitutional perspective in multiple ways".
Several ways in which the bill could undermine legal certainty were identified. The government's proposal to create a new category of retained EU law, for example, could cause "problematic uncertainties and ambiguities".
In addition, the parts of the bill that would give ministers the power to amend laws without full parliamentary scrutiny were "overly broad", peers said. Known as Henry VIII powers, the committee believed there was an "unacceptably wide" emergency procedure for short-term changes.
The report also contained a warning that failure to secure agreement from the Scottish and Welsh governments over the devolution of powers returned by Brussels could have "significant constitutional repercussions".
Although the bill faced hundreds of challenges in its passage through the House of Commons, the government suffered only one defeat when MPs insisted on a guaranteed vote on the terms of leaving before Brexit.
The legislation will be the subject of two days of debate in the House of Lords from Tuesday (30 January).
The Guardian reported that Labour peers were planning to support about 20 amendments in the Lords, which may be successful with the support of cross-bench and Liberal Democrat peers.
Key challenges include a vote for the UK parliament on the final deal ahead of the EU parliament, a time-limited transitional period on current terms and the removal of the government's fixed date for departure, the newspaper said.
A spokesperson for the Department for Exiting the European Union told IBTimes UK, "The purpose of this bill is to ensure the UK has a functioning statute book on exit day. It is an essential piece of legislation in the national interest."
The committee's report would be carefully considered, the spokesman added.
"From the beginning we have been committed to working collaboratively with parliamentarians to improve the bill wherever possible. We will continue to do so in the Lords," the spokesman said.