The Conservatives are keen to seize on an area of policy where there is clear blue water between them and Labour. While David Blunkett, Labour’s education spokesman, has sought to reassure grammars that they face no threat to their continuance, their ethos or their quality from Labour-controlled local education authorities, he has also confirmed there would be no extension of selection under a future Labour government.

During the election campaign, Conservative Central Office will stress that the party is committed to encouraging new grammars and allowing existing schools to increase the level of selection.

The Education Bill currently under scrutiny in the Lords provides for grant-maintained schools to select up to half their intake without having to seek approval from central government. The Education and Employment Secretary, Gillian Shephard, is to get new powers to overrule local authorities attempting to block grammar proposals from their schools.

In recent interviews, Mr Blunkett has not significantly changed the party’s approach to selection; he has sought to make clear that a Labour government would not give the go-ahead to local authorities to begin the procedure for abolishing grammars.

The party’s revised policy on grammars appeared in June 1995 in the document, Diversity and Excellence, which was Labour’s attempt to grapple with the future of grant-maintained schools, given that the party’s leader was planning to send his son to such a school.

That paper raised for the first time the prospect of parents in areas with grammar schools being balloted on abolition. It said: “While we have never supported grammar schools in their exclusion of children by examination, change can only come through local agreement. Such change in the character of the school would only follow a clear demonstration of support from parents affected by such decisions.” At that point, Mr Blunkett suggested the details of the procedure could be left until a Labour government had been elected, but that Labour would provide for ballots of parents with children in primary schools in the catchment areas of grammars.

That autumn, Roy Hattersley’s attack on the party’s policy on grant-maintained schools, which he maintained would lead to a two-tier system in schools, led to Mr Blunkett’s famous response: “Watch my lips, no selection by exams or interview under Labour.”

Since then, the ballot procedure for the abolition of grammars has come under intense scrutiny in the Wirral by-election campaign. The constituency has two grammar schools. Labour’s critics insist there will be problems defining feeder schools in areas where grammars draw from a wide distance.

Labour this week was saying a ballot would only be held where a request came from “a significant” number of parents - it might be of the order of 20 per cent of the parents in primary feeder schools.

The drawing-up of catchment areas will present problems. Labour suggests that in areas where there is selection, such as Trafford or Buckinghamshire, all parents of school-age children could be balloted.

For Mr Blunkett, the media concentration on the future of 161 schools is diverting attention from Labour policies on raising achievement. However, the future of grammars is likely to be a local issue in at least 12 marginals. The Conservatives are also determined it will be a national debate with promises of grammar-school streams in schools and more grammars.