Deputy PM Nick Clegg has come under fire for planning a voting system referendum on the day of elections in Scotland, Wales and Northern Ireland.
He wants the vote on switching from first-past-the-post on 5 May 2011.
A succession of Tory MPs criticised the planned date and Plaid Cymru's Elfyn Llwyd said it smacked of "contempt".
Mr Clegg said holding it on the same day would save millions and voters would be asked an "uncomplicated" yes or no question.
In a wide-ranging statement on electoral reform he announced plans for fixed-term parliaments, plans to cap the number of MPs at 600 - 50 fewer than now - and to review constituency boundaries to create "fewer and more equally sized" seats.
But it was his decision to set a date for a referendum on changing the voting system to the alternative vote (AV) which upset some MPs.
The referendum was a key part of the coalition deal between the Lib Dems, who want to change the voting system, and the Conservatives, who back first-past-the-post.
Their MPs will be ordered to support a referendum when legislation comes to the Commons - but they will be free to campaign on opposite sides of the debate.
Several Tory MPs stood up to criticise the decision to hold a referendum on 5 May.
The former shadow home secretary David Davis warned it would lead to "differential turnouts which means that the subsequent referendum is unrepresentative".
Tory MP Bernard Jenkin, who is expected to play a leading role in the "no" campaign in any referendum, said that while changing the voting system was a "big deal" in Westminster, it was of "scant interest" to most people.
He suggested Mr Clegg was "artificially inflating the turnout" - without any advice from the Electoral Commission.
Conservative MP Gavin Barwell added that the date "could lead to a skewed result" as people in areas where there were no elections that day would be less likely to turn out to vote in the referendum.
For Plaid Cymru, Mr Llwyd said it ignored the finding of previous reports and "sounds to me not like the respect agenda but actually the contempt agenda".
DUP MP William McCrea complained there were already two sets of elections in Northern Ireland on that day - the Assembly and local government elections - and asked if they were expected to move one.
Speaking for the SNP after the debate, Scotland's Deputy First Minister Nicola Sturgeon said SNP ministers would try to block the "utterly unacceptable" date.
She said: "There is a real danger of the Scottish Parliamentary elections being overshadowed and the issues that really matter to the people of Scotland being eclipsed."
Lib Dem silence
Labour's Austin Mitchell said it was a shame Mr Clegg did not have the "guts" to stick by his desire for proportional representation - and said AV would only benefit the Lib Dems.
Shadow justice secretary Jack Straw said none of the four previous referendums in the UK had been held on the same day as an election.
He added: "Would it not be altogether more sensible to consult widely on the best possible date and then add the date to the Bill in due course?"
Mr Clegg said the referendum would be on a simple "yes or no" question at a time people were voting anyway - and it was "patronising" to suggest people could not make more than one decision on the same day.
He argued it would cost millions to hold a referendum on another day and "additional cost, complexity and delay" would arise.
But BBC political correspondent Norman Smith said the silence among Lib Dem MPs was astonishing - with only one or two getting up to back Mr Clegg - something he said would be deeply worrying for the deputy PM.
There were also hints that Tory MPs might try to introduce amendments to ensure the referendum would only be binding if at least 40% of the electorate took part.
AV was not the Lib Dems' first choice - they would prefer the Single Transferable Vote, a more proportional system.
Lib Dem MP Mike Hancock told BBC Radio 4's PM programme it was "second best and for that reason I think a number of people are not that enthusiastic about it". But he added: "At least it's a step in the right direction".
He also said he would have preferred the referendum to have been later.
Under the proposed AV system, voters rank candidates in order of preference.
Anyone getting more than 50% in the first round is elected, otherwise the candidate with the fewest votes is eliminated and voters' second choices allocated to those remaining. This process continues until a winner emerges.