Lord Holme, the party's general election campaign manager, said that legal opinion indicates that the any attempt to bar Mr Ashdown would contravene the BBC's charter or the Broadcasting Act for independent companies.
Although broadcasters have been conducting lengthy negotiations with the political parties, none has so far clinched a deal to conduct a debate.
Mr Major is expected only to agree to one at the eleventh hour if his poll rating stays poor and he judges he has nothing to lose. But the Conservatives are unhappy at the prospect of Mr Ashdown taking part because they believe that he will direct most of his criticism against the Government, rather than the Opposition. That raises the possibility of a two-on-one confrontation.
Lord Holme said that the legal advice was "100 per cent clear" that the one-to-one debate could not take place. If a TV company tried to go ahead the party would have recourse to judicial review, he said. However, he added that the party would probably not be able to demand equal airtime.
The Liberal Democrats are, he said, "realistic enough to recognise that, at the margin there would have to be some tinkering". That would probably follow the formula for most TV discussions where Liberal Democrats usually get given slightly less time than the two main parties.
The Liberal Democrats argue that the same rule does not apply to the Referendum Party, funded by the millionaire tycoon, Sir James Goldsmith as the existing rights to airtime are determined by performance at the last general election.
The Liberal Democrats will next month seek to boost their profile with the electorate by producing a party political broadcast featuring comedian, John Cleese who will ask voters to send postcards pledging their support for the party.