Peers have criticised the government's attempt to fix the length of parliamentary terms, saying the proposals owe more to "short-term considerations than to enduring constitutional principles or sustained public demand".

The government is attempting to pilot through legislation that would bring in five-year parliaments, due shortly in the Lords for its second reading. Ahead of this, the Lords constitution committee says in a report published today that they are "unconvinced" by the arguments for moving to a fixed-term parliament.

The group includes constitutional expert Lord Norton; former cabinet minister Lord Crickhowell; Lord Renton; Lord Powell, adviser to Margaret Thatcher in Downing Street; former Lib Dem leader in the Lords, Lord Rodgers; and the former lord chancellor Lord Irvine.

They take issue with the idea that the bill will make parliament more accountable to the public as they say fixing elections at five-year points would lead to fewer elections. Since 1945, the peers say, there have been 18 governments that have lasted an average of three years and 10 months. They conclude five years is too long and governments should go for four-year fixed terms.

Nick Clegg appeared before their committee to defend this point, saying a four-year parliament would not give an administration time to govern in the national interest. He said governments tended to be "hamstrung and paralysed" for at least a year before a general election. A five-year term, he said, would allow four years to "get on and do difficult things". The deputy prime minister said: "It's a combination of providing a length of time with which people are familiar and which allows governments at least maybe four of those five years."

The Lords said the five-year term was "inconsistent with the government's stated aim of making the legislature more accountable, inconsistent with existing constitutional practice and inconsistent with the practices of the devolved institutions and the clear majority of international legislatures".

Lady Jay, chair of the committee, said: "The majority of the committee could see no justification for the period of fixed-term parliaments being set at five rather than four years. The move will reduce the frequency of general elections, and reduce parliament's accountability to the public, not increase it, as the deputy prime minister has stated is his aim. In our view the government have failed to make the case for such a significant constitutional change and undertook no consultation or pre-legislative scrutiny before they bought the legislation to parliament. That is extremely regrettable.

"We conclude that it is sensible for the bill to contain some form of safety valve, which would allow for an early election in circumstances such as the government losing the confidence of the Commons or where a political or economic crisis has affected the country."