The cabinet has been warned that Scottish nationalists could win the independence referendum next year, by tapping into Labour-inclined voters who have never forgiven Margaret Thatcher for the rapid collapse of Scottish manufacturing.

Amid concerns that ministers in London are adopting a complacent attitude towards the referendum, the Scotland secretary, Alistair Carmichael, "put the fear of God" into the cabinet by warning that the referendum could easily be lost by supporters of the United Kingdom.

In an interview with the Guardian, before publication by the Scottish government next week of a white paper on its plans for an independent Scotland, Carmichael said: "I am convinced that this is a fight that can still be lost … winning with a 49-51 split actually resolves nothing. Do not assume we will get the 51% – we could end up with the 49%."

In an attempt to show that the pro-UK Better Together campaign is taking nothing for granted, Carmichael has intensified the pressure on Alex Salmond, Scotland's first minister and leader of the SNP, by mocking its plans to form a sterling "currency union" with the remaining part of the UK after a victory in the referendum.

The Scotland secretary, who is the Liberal Democrat MP for Orkney and Shetland and who was drafted into the Scotland Office in the recent ministerial reshuffle to make the UK government's referendum campaign punchier, told the Guardian: "The general approach [of the SNP] is to de-risk what they are offering. So on the currency they used to want the euro, now they want a currency union with the UK. They want to walk away from the UK but they want to keep the currency, the Bank of England as the lender of last resort and [the capacity to build] warships on the Clyde."

Carmichael added: "Alex Salmond has said he wants and will insist on a currency union. The only way you can insist on a currency union is to remain part of the UK. George Osborne and Ed Balls have both said that a currency union is highly unlikely. We know from our friends in the eurozone that a currency union is difficult to make work. So the first question to Alex Salmond is: if you do not get a currency union what is plan B?"

Carmichael also pointed out that the provision of pensions in an independent Scotland would be very expensive because as an independent nation it would have the "acute" problem of an ageing population. By 2060 every pensioner in Scotland will be supported by 1.9 people of working age compared with 2.2 in the rest of the UK, according to the Treasury chief secretary, Danny Alexander.

The Scotland secretary also said that Salmond must come clean on the costs of establishing an independent governmental system. A leaked memo from the Scottish finance minister, John Swinney, said the running costs of a tax system would be £600m.

Carmichael spoke out amid fears that political leaders in England are assuming that unionists will easily win the referendum next September. The Herald reported a source close to David Cameron saying in July that the pro-union Better Together campaign has already won the referendum and now simply needs to secure a large majority to put the issue of independence to bed.

Carmichael decided to warn the cabinet because of concerns at what is seen as complacency in England, as opinion polls show enduring support for the union. A TNS BMRB opinion poll for Scotland on Sunday this month found that 47% supported the union while 29% supported independence. It found that nearly a quarter of voters (24%) were undecided.

Carmichael told the cabinet that undecided voters might be tempted to vote for independence. The turnout is expected to be as high as 90%, meaning that voters who currently say they are undecided could number 1 million on polling day.

He told the Guardian: "See how many undecided people there are. They are formerly reliable Labour-voting males in the urban post-industrial belt of Scotland. That is Glasgow, Fife Lanarkshire, Ayrshire. They have a folk memory. They remember Margaret Thatcher and the de-industrialisation of Scotland in the 1980s. They were not overly impressed by 13 years of Labour government and as a result their loyalty to Labour has loosened. They went lock, stock and barrel to the nationalists in 2011.

"There may be 1 million people likely to vote that have never voted before; they have not been reached by conventional politics."

He hoped the pro-union campaign would be strengthened by voices from across England and Wales. He praised Carwyn Jones, the Labour first minister of Wales, who warned that Cardiff would probably try to block Salmond's plans for a currency union. Jones said in a speech in Edinburgh: "I would be uncomfortable being part of a currency union where there are competing governments trying to run it. If there is a disagreement, who has the final say? This is a recipe for instability – and these things matter, particularly in times of crisis."