Nick Clegg has admitted that the Scottish people will not be given a chance to vote on their main preference in the independence referendum that is to be held in just over a year's time.

As David Cameron issued a warning that a vote for independence would lead to a "leap into the unknown", the deputy prime minister said that greater devolution "isn't exactly on the ballot paper".

Speaking in Glasgow on the second day of the Liberal Democrat conference, Clegg said there was a consensus in Scotland for "devo-max" – handing most powers to the Scottish parliament bar defence, foreign policy and sterling.

Clegg told The Andrew Marr Show on BBC1: "I certainly think in many ways actually what the Scottish people want isn't exactly on the ballot paper – which is a greater expression of Scottish nationhood, greater devolution of powers from London to Holyrood, what is called in the jargon "devo-max" or in Liberal Democrat language, ever since the days of Gladstone, home rule. In many ways that is where I think we need to go as a nation – as a United Kingdom and as Scotland."

But Clegg indicated that the Scottish people would be given a second vote, to allow them to decide on "devo-max", if they vote to remain in the UK in the referendum that will be held a year on Wednesday.

He said: "You can't have that discussion on which powers you devolve until you first determine that Scotland remains part of the UK."

The remarks by Clegg are likely to be seized on by nationalists who had believed that the people of Scotland should be given two choices in the referendum – independence or "devo-max". But George Osborne ruled this out and pressed for a straight in-out vote after calculating that the electoral success of the SNP denoted anger with Labour, the largest party in Scotland in recent decades, and not overwhelming support for independence.

The prime minister issued an overnight plea for the people of Scotland to remain in the UK. In a statement issued on the eve of the 365-day countdown to polling, Cameron said 18 September 2014 was "Scotland's date with destiny. It will be time for Scotland to decide. The best of Scotland and the best of British or a leap into the unknown? Scotland's future is in Scotland's hands."