Nationwide, Britain’s biggest building society, is urging those who want to benefit from the government’s stamp duty holiday to act “very quickly” to secure a mortgage.

The stamp duty threshold rose in September 2008 from £125,000 to £175,000. However, it will return to £125,000 on December 31 unless the government extends it. The government is unlikely to confirm this before the pre-budget report, expected this month.

In a note to brokers, Nationwide said: “There are only 42 working days until the threshold returns to £125,000. As the average Nationwide purchase application takes around 39 days, you’ll need to get these applications in soon if your customer wants this discount.”

Last week, Nationwide announced a range of deals for borrowers with deposits of less than 10% — including a two-year fix at 5.98% with a £495 fee, provided you have, or open, a Flex current account with the society.

The deals came as Nationwide reported a 0.4% rise in the average house price to £162,038 last week — a slower rate of growth than in the previous three months.

Martin Gahbauer, Nationwide’s chief economist, said the mini-boom showed “some signs of moderating”.

Aaron Strutt of Trinity Financial Group, the broker, said: “Previously, Nationwide only offered good deals to existing customers when it came to those with deposits of less than 15%, so this suggests some growing confidence in the housing market.”

Nationwide said it could not guarantee that mortgage applications initiated now would be completed within 39 days, but said: “We will do our best to help cases that are urgent.”