Gordon Brown will be staring at a black hole in public finances by 2007, the Liberal Democrats have claimed.

The party claimed on Friday that the chancellor will have to fill a £15 billion spending hole due to a shortage of taxpayers.

Brown's predictions of how may people will be working to pay tax, which he set out in the last budget, is based on the population growing by more than 2.25 per cent over the next 20 years.

But the Liberal Democrats claimed another government study - published on the same day - predicts a million fewer people by 2020.

But a study commissioned by Brown and published on the same day predicts one million fewer people than he estimates by 2020.

The party claimed figures released by the Actuary Department forecast a 2.4 million gap the group of people aged 15 to 64 who are the main taxpayers.

With fewer people paying in and more elderly people relying on the NHS, the Liberal Democrat treasury spokesman Matthew Taylor predicted that as a result the chancellor will need to find £5 billion for the 2006-7 financial year.

"To keep his spending plans prudent, Gordon Brown assumed higher population growth in his last Budget, and so increased forecast long-term growth trends," he said.

"Brown chose figures to suit his spending needs. Short-run economic problems won't blow the Chancellor off course - but having to cut his long-term growth forecast because of population overestimates opens a black hole in the Budget which will grow by billions every year."