Within this, residential sales fell 12.9% year-on-year, from £8.6bn in 2016 to £7.5bn in 2017.
Meanwhile, buy-to-let (BTL) sales also saw falls, dropping by an annualised 12.1%, from £2.1bn in December 2016 to £1.8bn in 2017.
In total, mortgage sales in December 2017 amounted to £9.3bn, against the £10.7bn in 2016.
Although December is traditionally a slow month for sales – owing to the festive period and seasonal effects – John Driscoll, director at Equifax Touchstone, said that the annualised decreases are nevertheless worrying.
“After three months of consecutive growth, mortgage sales in the UK have decreased sharply across both residential and BTL sectors,” said Driscoll.
He added: “The level of decrease is somewhat concerning for the industry, especially considering that mortgage sales are down £1.4bn year-on-year.”
Driscoll continued: “While we expect to see the usual New Year pick-up in the market following a festive dip, there are a number of factors at play which could alter the direction of mortgage sales in coming months.
“An uncertain economic and political outlook, the onset of Open banking and whether this will facilitate faster mortgage applications, the end of the Term Funding Scheme and implication for higher mortgage rates, and the subdued forecast for house prices – to name a few – have set the scene for a volatile and uncertain market in 2018.”
The average value of a residential mortgage during December 2017 was £191,522, against £196,682 in the same period the year before, while average BTL mortgages in December 2017 stood at £150,914 against £158,967 in the previous year.