It comes amid clear signs, in a series of ministerial interviews, of sharp divisions within the Cabinet over whether a referendum either on a single currency or further EU integration would be desirable.
The referendum idea could be among issues to be discussed this morning by ministers at a top-level local government elections post-mortem at Downing Street.
The decision to hold out the prospect of a national ballot in the Liberal Democrats' manifesto is revealed in a letter, leaked to the Independent, from Mr Ashdown to Tim Clement Jones, the party's European campaign director. It was written this weekend in the wake of the party's poll successes.
With the Liberal Democrats threatening to overtake the Tories in the European elections, the outcome of which could yet determine Mr Major's survival as Prime Minister, the move intensifies the Government's dilemma and could outflank the Conservatives by making it more difficult for the leadership to justify denying a referendum.
In his letter, Mr Ashdown indicates the decision to include the pledge in the manifesto, agreed in secret by the party high command last month, is in line with policy endorsed at the party's conference last year of 'involving people in major decisions' by referendums if necessary.
He adds: 'We must stress that we would wish to obtain public assent for further major constitutional change for very different reasons than the Tory right. They wish to suppress and distort the real facts about Europe, play on prejudice and secure a mandate for isolation and decline.' Emphasising his party wants to 'win the people's support,' Mr Ashdown adds of the forthcoming election campaign: 'I do not wish to hear any campaigner using this to say 'this is your chance to say no'. We should be saying 'You have a right to say, but we want you to say yes'.'
He says: 'We must not make the Maastricht mistake again, in which we nearly lost the whole project to advance European Union because we failed to explain its relevance to ordinary people.'
Although the move, which follows Mr Ashdown's support from a pro-European stance for a referendum on Maastricht, explicitly applies only to the 1996 conference, it could allow for one on a single currency, though he regards that as impractical. He is expected to emphasise that 'public assent' for the conference's decisions could be secured by making them the central issue at a general election. Assuming that does not happen, a referendum is the likeliest alternative.
Kenneth Clarke, the Chancellor, yesterday came closest to ruling out a single currency referendum, saying on BBC radio that he was 'not at all attracted by the idea'. John Redwood, right-wing Secretary of State for Wales, said on BBC TV's On the Record: 'I'm not saying yes or no to a referendum. I'm saying you have to wait and see when you get a big enough issue.'
The depth of the split over Europe will be underlined in a BBC Panorama programme tonight when Tim Renton, former party chief whip, describes it as the 'San Andreas fault' of Tory politics. He says in 1996 the party 'will have to consider very seriously whether it can stay together over the European issue or not'.
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