A political row has broken out over lobbying surrounding the Government’s decision over controversial plans for the rail freight depot on the former Radlett Aerodrome site.

St Albans MP Anne Main has written to cabinet secretary Jeremy Haywood after Chipping Barnet MP Theresa Villiers is said to have broken the ministerial code, by failing to report the potential conflict of interest during her time as Minister for Transport in 2011.

The St Albans MP, who fiercely opposes the plans for a huge rail freight terminal on green belt land, claims Ms Villiers subsequently used her influence to inform what she calls a "puzzling" change in heart by secretary of state Eric Pickles. At the time of the alleged breach, developer Helioslough had re-submitted previously rejected plans.

Ms Villiers met with Helioslough’s chief lobbyist and old university friend Simon Hoare for lunch on August 10, weeks before the application was due to be reassessed by the secretary of state.

Mr Hoare followed up the meeting with an email requesting the support of Ms Villiers’ office in achieving "a positive result" on the decision.

Mr Pickles had previously rejected the bid but when it was sent back to him following a judicial review in July 2011, he invited submissions from interested parties.

The plans are still with his department and Mr Pickles recently said he was "minded to approve" the application, despite mass protests from politicians and campaigners.

Mrs Main wrote to Mr Haywood on September 10 requesting a full investigation of the alleged breach of the ministerial code, which she says could have a "potentially enormous consequences" for her St Albans constituents.

A spokesman for Mr Haywood confirmed his office had received the letter and said: "We will respond in due course."

Mrs Main said: "It is with great sadness that I have had to take this step, but I really must do the best for my constituents and I am left with the lingering feeling that there has not been a level playing field.

"My constituents certainly did not have the opportunity to have private lunches with the minister to put their point of view across.

"I see no other reason for Mr Hoare to meet with Ms Villiers other than to make a persuasive argument and bend the ear of an old friend.

"The ministerial code is in place to reassure the public that this cannot be the case. Lobbying behind the scenes gives a very bad impression to the public, and on this matter my constituents smell a rat."

Ms Villiers defended her decision.

A spokesman for Theresa Villiers said in a statement this afternoon: "Any rail minister will have matters relating to rail freight raised with them from time to time.

"The conversation with Simon Hoare was published to Parliament in an answer to a parliamentary question.

"Nothing from that conversation was passed on to ministerial colleagues. The meeting therefore had no impact on consideration of the Radlett planning application.

"These matters were all thoroughly ventilated in a series of correspondence with Mrs Main well over a year ago."