The company, Decision Makers, was heavily involved in the campaign for Ebbsfleet, Kent, to be the international station instead of Stratford in east London.
A spokesman for Dame Angela said she had decided to resign following a London Evening Standard report this week that the company had helped secure the station for Ebbsfleet. She had declared the post in the Register of Members' Interests, had nothing to hide, and would be making no further comment.
The move came after Labour reported her to the Commons Select Committee on Members' Interests earlier this week for what it claimed was 'apparently grossly improper behaviour'.
Brian Wilson, Labour's industry spokesman, called for an urgent statement from the Prime Minister, describing the matter as 'very serious'.
The newspaper claimed that the cement company Blue Circle, which owns the land chosen for the site of the station, had employed Decision Makers to press its case with the Government. Dame Angela declared Blue Circle Properties as a client of Decision Makers in the register.
Members of the lobbying company allegedly met a series of Government ministers, including then Transport Secretary John MacGregor and even the Prime Minister. Some of the meetings were reported to have taken place over lunch or dinner or at social events such as an evening with ministers at Hampton Court.
The decision to build the station at Ebbsfleet was announced in the summer by Brian Mawhinney, Secretary of State for Transport. A source close to Dame Angela said that she and the company had decided that the 'hassle' was no longer worth it.
Dame Angela appears to be an assiduous recorder of her interests, listing in the 1993 register 12 clients of Decision Makers as well as overseas visits and five unpaid positions.
But Mr Wilson said: 'Coming on the heels of the Ebbsfleet story, Dame Angela's resignation appears to represent an admission that a conflict of interest has existed. In the light of that, there should be an early statement from the Prime Minister on just how much influence she wielded in advance of the Ebbsfleet decision with its enormous financial implications.'
Dame Angela's abrupt resignation from her post at Decision Makers is a rare blip in an otherwise textbook political career free of banana skins.
A London University history of art graduate, she joined Parliament as MP for Mitcham and Morden after a 1982 by-election.
While resigning her membership of the right-wing Monday Club two years later, she remained on the 'dry' side of the party.
The one exception from that approach is the backing she has given for tax relief for child-care costs. A working mother and former chair of a ministerial group on women, she and her husband John have two sons and one daughter.
A beneficiary of John Major's pledge to give women higher profile roles, the 61-year-old MP for Mitcham and Morden, was moved from education to the Home Office as minister of state in 1990.
The brief included prisons, coinciding with the Strangeways riots, dangerous dogs and Sunday trading.
She was rewarded in 1992 with her election to Dame Commander and deputy chairmanship of the Tory party. She is now in charge of the key task of recruiting new Parliamentary candidates.
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