Hillsdown Holdings, which owns Buxted Chickens, undertook the work as part of a two-day mini-agricultural show organised for European farm ministers as part of the EC presidency at Mr Gummer's Suffolk home in September.
The Ministry of Agriculture said the work on the badly silted pond was intended as a demonstration of environmental restoration for the show. Adas, the farm advice agency, drew up the specification for the work, but did not charge Hillsdown.
The Prime Minister's office and the Ministry of Agriculture insisted last month when the improvements became public that there had been no impropriety by Mr Gummer. Parliamentary answers show Hillsdown, one of Britain's biggest food companies, spent more than pounds 2,600 on the landscaping and restoration of the pond and a marquee at the show.
Officials said the agriculture minister had ended up out of pocket in hosting the show for nothing in a field alongside his restored rectory home near Debenham in his constituency.
A Labour MP is now understood, however, to have complained to the members' interests committee that Mr Gummer failed to declare what many Labour MPs judge to be a gift.
The new register, published this month, lists interests as of 1 December, recording gifts going back to January last year. Mr Gummer has not made any entry since 1 December.
He said: 'I have not obtained any advantage as a result of the visit of my colleagues the EEC agriculture ministers.'
The show was held in September. Under the rules, 'it is the responsibility of members to notify changes in their registerable interests within four weeks of the change occurring'. The rules require MPs to declare any material benefit 'which a member may receive which might be thought to affect his conduct as a member or influence his actions, speeches or vote in Parliament'.
Under normal procedures, because the complaint has come from an MP, it will automatically be referred to the committee. It is likely to write to Mr Gummer seeking his explanation, before deciding whether to hold an inquiry.
The fresh allegation that a Cabinet minister has failed to declare a registerable interest comes as the committee will, on Monday, report on Norman Lamont's acceptance of pounds 18,414 from Conservative Central Office to meet the legal costs of evicting a 'sex therapist' from his London home.
Mr Lamont has argued he did not need to register the money because it was given anonymously, and thus could not influence his actions.
The committee is understood to criticise Mr Lamont for not consulting the registrar of members' interests on whether the cash should be declared, and to recommend a clarification of the rules to make it clear that anonymous donations should be registered.
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