At the party's conference in Llandudno on Saturday they threw their weight behind a petition demanding that census forms contain a Welsh tickbox.

In the motion, Plaid's National Executive Committee condemned the Office for National Statistics for not allowing Welsh as a description.

The motion said the census will be flawed as a result and attacks the ONS for "lack of consultation" and "arrogance" in refusing to amend the form.

'Insult to Wales'

It said the contrast with Scotland - where residents may describe themselves as Scottish and so reject the description "British" - underlined the need for the National Assembly to have primary law-making powers and its own National Statistics Office.

It is the latest round in a growing campaign to get the ONS to give people in Wales the same opportunity as people in Scotland to register their nationality.

In the 2001 census, there will be no special box for people to tick in order to classify their nationality as Welsh.

Welsh people will only have the option of classifying themselves as British, Irish or Other.

Gwynedd council has described the ONS's attitude as an insult to Wales.

The ONS has so far refused to include a Welsh tick box, saying that if people want to class themselves as Welsh they can tick the box marked "Other" and then write "Welsh" alongside.

This has not proved good enough for protesters in Wales, and there have been increasing calls for Welsh people to be given the same status as the Scots when the census is carried out next year.

Council leaders join campaign

The Welsh Local Government Association is among those who have called on the ONS to include a Welsh tick-box.

Plaid Cymru president Ieuan Wyn Jones has written to Mr Blair seeking a pledge from the government to make changes to the recently-published forms.

"People are appalled that the Office for National Statistics (ONS) is refusing to change its mind on the issue," said Mr Jones.

"I think Tony Blair should intervene and sort this out.

'Discriminatory'

"As there is to be a 'tick box' for the Irish and Scots, people simply cannot understand why there cannot be one for Wales.

"This is grossly discriminatory and unfair."

The ONS said census legislation for Scotland was devolved to the Scottish Parliament, which had taken the decision to add the Scottish tick box.

For Wales, the matter was decided in London, and the decision was taken not to include a tick box for people to say they were Welsh.