The MLA also disclosed that the rental expenses he and First Minister father claim from Stormont for the Ballymena premises add up to "at most" £56,000 per year.

He disputed weekend reports that their combined total was £62,000, but the figure is still significantly higher than sums claimed by other MLAs.

As the Belfast Telegraph reported on Saturday, the office at 9-11 Church Street in the town is owned by a company called Sarcon.

Developer and DUP member Seymour Sweeney was the sole director of this firm last summer, when the office was being purchased.

Mr Paisley Jnr last night said Mr Sweeney was replaced in 2007 as sole Sarcon director by Lisburn man James Currie - the junior minister's father-in-law.

It is permitted within the Assembly rules to claim rental expenses for constituency offices owned by family members. The House of Commons halted this practice a number of years ago.

Mr Paisley said: "There is no profit for Sarcon from the rental payments. It goes directly to the bank to furnish the mortgage."

The MLA also defended the rental level. He has stated that it was set according to market value, as advised by a professional agent, and said the arrangement was "validated" by the Assembly.

The premises include a meeting place for 150 people which is used as a community facility, he added.

"There is also a suite of offices, a general office, a general public area, kitchen and storage facilities and private car parking to the rear," he continued.

"I dealt with 18,500 constituency cases last year. I imagine that is more than any other Assembly member.

" To provide that level of service, you need good office facilities."

The junior minister said that the running of the building is overseen by a management trust made of local DUP members.

There were signs over the weekend that support for the junior minister was falling within the DUP at the Assembly.

His position has previously been viewed as very strong, given the unwavering support he receives from his father.

But speculation at Stormont is growing that others in the party have lost patience.

The junior minister last night said: "I have been under a lot of pressure and received undue attention for issues that also affect other MLAs. I take it on the chin and move on."

Mr Paisley Jnr's father-in-law, James Currie, is a retired company director who received an MBE in the New Year Honours.

He and his Paisley Jnr both own properties in the Ballyallaght development near Bushmills built by Mr Sweeney.

It has been stressed that they both paid full market value for the properties.