The Portman has kept up the pressure on its rivals in the building society sector, announcing its assets rose by 15 per cent to £17.8 billion last year.

Portman, which merged with the Wolverhampton-based Staffordshire in 2003, is the country's third biggest mutual with 1.8 million members, 142 branches and 2,300 staff, and claims to be the fastest growing of the top ten.

Annual figures released by Portman showed that growth was seen across the business in 2005, with residential mortgage balances up 16 per cent to £13.5 billion and residential-based lending ahead by 18 per cent to £4.3 billion.

Chief executive Robert Sharpe said it had been a successful year for the group, particularly in light of challenging trading conditions.

While Portman is run for the benefit of its members, the group achieved its ninth consecutive year of profits growth with a rise of three per cent to £85 million.

Developments during the year saw the company expand sales and administration functions at the Staffordshire which is now contributing to the Portman Direct operation.

The group has also established a financial planning sales force, expanding the breadth of service offered to members.

As a result investments made through the society increased by more than 50 per cent compared with 2004.

Although Mr Sharpe said the industry faced economic uncertainty in 2006, he was confident Portman would be able to build on its market position.

"Growth will continue to underpin our success as a mutual organisation and our focus remains firmly on providing a strong infrastructure to support this strategy," he added.

The group, which was established in 1846, has more than doubled in size since 2000, mainly through organic growth with the development of products such as Portman Direct.

The takeover of Sun Bank in 2000 and the Staffordshire Building Society in 2003 accounted for nearly a third of growth.

Meanwhile, the much smaller Tipton & Coseley - the 40th biggest building society - was also able to highlight some impressive annual figures.

Net savings rose by an impressive 80.27 per cent to £16.26 million and group assets increased by 9.76 per cent to £287.43 million.

The T&C has been gaining business recently from customers deserting from Birmingham Midshires - the former building society that is now part of the giant Halifax/Bank of Scotland financial services group - following the decision to shut its high street branches.

It said the performance of its four branches had contributed to 2005's record results.

Transaction levels rose by about 6.10 per cent and gross mortgage lending totalled £56.34 million.

There has also been a fourth successive reduction in the management expenses ratio to 0.98 per cent which will help sustain the competitive nature of the Society's mortgage and savings products.

Chief executive Chris Martin praised the society's employees for their contribution last year.

"We place the development of our workforce high on our agenda, believing well trained, well informed and knowledgeable staff play a fundamental part in providing exceptional levels of customer service," he said.