There are now strong signs of growing impatience among his DUP colleagues, amid revelations about the constituency office that the MLA and his First Minister father rent at taxpayers' expense in Ballymena.

It is costing more than any other MLA office, with the payments going to a firm controlled by Mr Paisley Jnr's father-in-law. The mortgage deal for the property was, meanwhile, secured by Mr Sweeney.

While making no public comment, DUP figures are said to be angry at the cumulative impact of the ongoing Sweeney-related disclosures.

One senior party source even said today: "There is no doubt that a head of steam is building up. I would say Ian Junior's position is precarious."

Other DUP Assembly sources were equally forthright, with one saying members were " sick to the back teeth" of the situation.

Another said: " There is a feeling that if it had been anyone else, they would have been out long ago."

As this newspaper reported on Saturday, Mr Sweeney secured the mortgage last July for the purchase of the constituency office by a company called Sarcon (no 250). He also provided free advice on the acquisition of the property at 9-11 Church Street in the town.

Control of Sarcon (no 250) - which is now getting its mortgage paid by Assembly funded rental expenses - was then transferred from Mr Sweeney to Mr Paisley Jnr's father-in-law, Lisburn man James Currie.

Mr Paisley Jnr last night said the annual rent he and Dr Paisley pay would amount to £56,000 "at most" and not the £62,000 reported at the weekend.

This sum is substantially more than totals claimed by other MLAs. According to draft figures circulated in the Assembly last week, the next highest rental claim is £20,000.

It has been privately admitted within the DUP that the "Junior factor" was raised by voters on the doorsteps during the Dromore by-election campaign which the party surprisingly lost.

Mr Paisley Jnr has faced sustained controversy over lobbying for Mr Sweeney on issues such as his Giant's Causeway visitor centre plan and a proposed purchase of prime publicly-owned land at Ballee, Ballymena.

Critics will now point out that the office mortgage deal was facilitated just weeks after the MLA had lobbied fellow Minister Margaret Ritchie on the Ballee site sale.

It also came less than two months before the furore surrounding Mr Sweeney's Causeway centre planning application - when Mr Paisley Jnr famously said " I know of him" about the businessman.

The junior Minister has continued to strongly defend his position. He has stressed that Mr Sweeney received no rental payments for the sizeable Ballymena office and was only briefly involved with Sarcon (No 250).

It is permitted within the Assembly rules for MLAs to claim rental expenses for offices owned by family members.

Mr Paisley Jnr 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, stating that it was set according to market value, as advised by a professional agent.

"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," he added.

There was speculation today that the latest row will be discussed at a DUP party officers' meeting.

Mr Paisley Jnr continues to enjoy the unwavering support of his party leader father and it would not be his normal style to go quietly.

It may also be significant that, while critics within the party are becoming more vocal, no one has so far spoken out publicly.

The MLA is by no means the only Ulster politician to have received rental expenses for an office owned by a relative.

As this newspaper revealed last week, Ulster Unionist Billy Armstrong until recently claimed rent for a prefab office owned by his wife and located on their farm.