Image caption The government aims to get the Bill on to the statute book by 16 February

Proposals being rushed through Parliament to equalise constituency sizes risk a "chaotic" redrawing of boundaries, a report has warned.

Changes would not have regard to local loyalties and historic ties, the think tank Democratic Audit said.

The government wants to cut the number of constituencies from 650 to 600, ending what it calls unfair voting.

The report warned that failure to iron out flaws in the seat equalisation plans could spark a "revolt" among MPs.

Report author, Lewis Baston, warned that ministers would "repent in leisure" their decision to combine the equalisation measures with the referendum on AV voting, in a single Parliamentary Voting System and Constituencies Bill.

The government aims to get the Bill on to the statute book by 16 February - possibly requiring all-night sittings in the House of Lords - in order to be able to stage the AV referendum on the scheduled date of 5 May.

Population shifts

If passed, changes would see an electorate in almost all seats within 5% of 76,000.

Nationally, wards which have traditionally made up the basic building blocks of electoral geography and party organisation will have to be split between constituencies.

It would be better to take a little longer and devise a system that will last Lewis Baston, Democratic Audit

It would mean that urban seats in cities like Doncaster and Coventry would have to take in countryside wards with few shared interests.

One constituency would have to unite areas in the Isle of Wight and Hampshire, the island of Anglesey would be joined to Bangor across the Menai Strait and a "Devonwall" seat would give one MP responsibility for parts of Cornwall and Devon.

Each constituency - with the exception of Shetland and Orkney, the Western Isles and the geographically massive Highlands seat of Ross, Skye and Lochaber - will have between 72,200 and 79,800 voters, under the proposals.

Ministers believe it would stop cases where constituencies have a large difference in the number of votes.

For example, the Conservative MP for the Isle of Wight, Andrew Turner, was elected by 103,480 voters, while the member for Arfon in north Wales, Plaid Cymru's Hywel Williams, answers to 42,998 people.

'Politically-driven'

The report warned that allowing just 5% deviation from the average would make it almost impossible to avoid constituencies crossing county borders.

Democratic Audit, an independent research organisation, calculated that all but nine counties in England would be forced to share a seat with a neighbour under the 5% rule, while none would have to do so if the margin was 10%.

Mr Baston said if the proposals were successful, it would require regular, disruptive changes to the electoral map to keep up with population shifts.

And he added that boundary commissioners should be given more leeway to make exceptions for island seats, rural areas and constituencies - mostly in inner London - where the number of residents hugely outstrips the number of voters.

"There are several fundamental issues with the legislation," he said.

"In particular the potentially harmful democratic implications of reducing the number of MPs, the increasing unreliability of electoral registration, and the hasty, non-consensual and apparently politically-driven way it is being implemented.

"But there are also a number of practical issues which make the government's proposals problematic.

"It would be better to take a little longer and devise a system that will last."