By Mark Devenport BBC Northern Ireland political editor

Tens of thousands of people could lose their votes in the next Stormont Assembly elections because of changes in the way the new electoral register will be compiled.

That is the fear being expressed by politicians on the eve of a major canvass by electoral officials.

The canvass will be supported by a high-profile advertising campaign.

But both nationalists and unionists remain concerned that their supporters might not get the message.

The drive to compile the new electoral register will be a major exercise.

Politicians worry that people could lose their votes

The Northern Ireland Electoral Office has employed almost 1,000 canvassers to knock on every door throughout Northern Ireland.

They start their work on Monday 9 September and the canvass will last for six weeks.

A register is compiled every year, but this time things will be different because of a new law designed to combat electoral fraud, which came into effect in May.

In the past, just one form was given out to each household.

Now every individual voter will have to fill out a form, supply their National Insurance number and date of birth and sign it personally.

Supporters

With about 1.2m voters in Northern Ireland, that is a lot of forms to process.

In the past, if a family forgot to return their form they were given a year's leeway before being deleted from the register.

Now it will be immediate, so not returning a form will mean not getting your vote.

With an assembly election due to take place in May 2003, political parties across the board are concerned in case their supporters miss out.

Alastair Patterson, an Ulster Unionist official who used to be a returning officer, says that if 10% of the voters in a constituency did not return their forms that would mean 7,000 votes lost - votes which could be crucial in a tight contest.

Mr Patterson believes the centre-ground parties could suffer more than their opponents, because it would be the less committed floating voters who do not bother to return their forms.

Michelle Gildernew: Said all parties could suffer

But Sinn Fein's Fermanagh South Tyrone MP Michelle Gildernew, who won her Westminister seat by just over 50 votes, says that all the parties could suffer.

She says measures designed to combat fraud could defer bona fide voters from registering.

All parties agree that groups which might be especially affected could be the elderly and students temporarily away from home.

The Electoral Office recognises there could be difficulties - and it points out that those who miss out will have another chance to get themselves on the register in the spring.

Whilst May is the government's proposed date for the assembly elections, the current instability means both the politicians and the electoral officers are braced for an early poll.

If that happens it could make it much more difficult for the authorities to correct any problems thrown up during the course of this autumn's canvass.