The preliminary inquiry is another potential blow for the Paisley family in a week that saw Ian Paisley Jnr dramatically resign as a Stormont Minister.

Mr Paisley Jnr today spelt out the nature of his Commons-related work for his father and said it involved a salary of £9,000 to £11,000 per annum.

That was on top of some £62,000 a year for his posts as a Junior Minister and MLA.

The preliminary Commons inquiry will be conducted by Parliamentary Standards Commissioner John Lyon, and follows a complaint from a member of the public.

Dr Paisley will be asked for his comments and can be expected to stoutly defend the pay arrangement.

Having assessed the situation, Mr Lyon will then decide if a full investigation is warranted.

Party colleagues have defended the departing Junior Minister on the Commons payments, pointing to the long hours he puts in on his political work.

"Ian Junior writes Dr Paisley's Commons speeches and does research on other parliamentary-related work," one DUP source said today.

The Commissioner's office told this newspaper yesterday that a complaint had been received on the matter.

It also stated that a separate complaint against Mr Paisley Snr from last year - relating to lobbying on a grant bid by developer Seymour Sweeney - is "still being considered".

The First Minister's son announced his ministerial resignation on Monday after months of controversy over his links to Mr Sweeney.

In the midst of this, he also faced questions over the fact he was still being paid from the Commons as a parliamentary assistant to his father.

In a statement issued to the Belfast Telegraph, Mr Paisley Jnr disclosed details of the Westminster-funded salary for the first time.

"Ian Paisley Junior MLA works as a research assistant to his father and has done so for 19 years," the statement said.

"The remuneration he receives is in the pay bracket £9-£11k per annum. His work includes research, advice and assistance with a wide range of Westminster-related constituency matters."

The father and son both represent north Antrim in the Assembly - the same constituency Mr Paisley Snr serves as MP.

The issue of politicians employing family members has been to the fore since Tory MP Derek Conway was suspended from the Commons last month.

A Standards investigation questioned his payments to his son Freddie, a full-time student in north-east England.

Politicians at Westminster and Stormont have stressed that it is entirely within the rules to employ relatives, provided they are doing the work.

Commons research assistants are paid up to £33,018 a year. Freddie Conway was receiving £11,773 per annum from his father for 17 hours a week.

A report by Mr Lyon's predecessor as Commissioner found that the rate was excessive in light of the student's experience and qualifications.

It also concluded that it was unlikely his work "was so extensive, or his availability in term-time was such, as to enable him consistently to meet his contractual commitment to an average of 17 hours a week".

The second parliamentary standards complaint against Mr Paisley Snr was made by SDLP MLA John Dallat last October.

It concerned an angry letter the DUP leader had sent to the Heritage Lottery Fund over a grant refusal for Seymour Sweeney's Giant's Causeway visitor centre scheme.

The 2003 letter - obtained by this newspaper through freedom of information - incorrectly claimed that the developer's plans had been approved by world heritage body Unesco.

An Assembly standards complaint was meanwhile submitted this week by SDLP MLA Declan O'Loan. It involved the £57,200 per year rental expenses claimed for the Paisleys' Ballymena constituency office, which is owned by a firm headed by Mr Paisley Jnr's father-in-law.

The DUP MLA has said the rent was set at market rate with advice from a professional valuer, and that the facilities of the office included a meeting room for up to 150 people.