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The government is to make a concession in an attempt to persuade MPs and peers to back its plans for a referendum on voting changes on 5 May.

Ministers will announce that they will protect the Isle of Wight from some of the boundary changes designed to ensure that parliamentary constituencies have roughly the same number of voters.

This was one of the key demands made by peers as the bill introducing the changes went through the House of Lords.

The Isle of Wight is a large constituency with some 110,000 electors. Under the government's plans, it would have been divided into two constituencies - but with one attached to the mainland across the Solent.

This prompted substantial opposition.

The government has now decided that the Isle of Wight will be divided into two seats - but within the geographic boundaries of the island.

On 19 January peers backed an amendment - by 199 votes to 122 - that would protect the Isle of Wight from being divided up. This was the largest defeat the Government had suffered in the House of Lords - there were 28 Tory and 14 Lib Dem rebels.

Government sources said they would now accept the principle of the amendment that was passed in the Lords.

The Parliamentary Voting System and Constituencies Bill would cut the number of MPs from 650 to 600 and change parliamentary boundaries so that constituency electorates are broadly the same - within a 10 percent leeway.

The bill already excludes two constituencies from the changes because they are geographically large and distinct: Orkney and Shetland, and Na h-Eileanan an Iar (the Western Isles).

Labour will accuse the government of bending the rules to create a new Conservative seat.