The reports said ex-transport minister Stephen Ladyman used his contacts
Lax rules are allowing former ministers to exploit contacts built up in public office for private gain, a committee of MPs has warned.
They are able to use such knowledge "with impunity" to lobby for causes from which they personally benefit, the Public Administration Committee said.
It is calling for tougher regulation of lobbying by an independent watchdog.
The committee's chairman, Labour MP Tony Wright, said: "Lobbying enhances democracy, but it can also subvert it."
He said: "Transparency is key here. There is a public interest in knowing who is lobbying whom about what."
A recent survey by the Committee on Standards in Public Life found that 56% of voters believed that at least half of all government ministers used their power for personal gain.
Ex-ministers are currently required to consult the Advisory Committee on Business Appointments when taking jobs related to their former responsibilities.
But the Public Administration Committee (PAC) investigation found that the advice given was far from consistent.
"We are strongly concerned that, with the rules as loosely and as variously interpreted as they currently are, former ministers in particular appear to be able to use with impunity the contacts they built up as public servants to further a private interest," its report said.
"We think that this is unacceptable, particularly where they continue to be paid from the public purse as sitting Members of Parliament. The rules need to reflect this."
The report cited the case of former transport minister Stephen Ladyman, who was subject to a lobbying "ban" after leaving his post.
Nevertheless, the report said, he was still able to arrange meetings with senior officials for when it ended and use his former office "as a way of introducing himself when lobbying" for a traffic information company.
The PAC said there should be a strictly enforced ban of several years preventing former ministers "from using the contacts and sensitive information that they acquired in public office to further their own and others' private interests".
The MPs also dismissed current self-regulation by lobbying firms, pointing out that just three complaints had been investigated in a decade.
"In the current climate of public mistrust, voluntary self-regulation of lobbying activity risks being little better than the Emperor's new clothes," they said.
The report called for the creation of a compulsory and independently monitored register of who was lobbying the government about what and what their interests were.
Any lobbyists breaking the rules should be stripped of their membership, it added.
"Lobbying the government should, in a democracy, involve explicit agreement about the terms on which this lobbying is conducted," the committee said.
"The result of doing nothing would be to increase public mistrust of government, and to solidify the impression that government listens to favoured groups - big business and party donors in particular - with far more attention than it gives to others."
It added: "There seems to be a culture of secrecy in some parts of government beyond that which is strictly necessary, and beyond that seen in some other countries."