The money was used to meet the cost of evicting a 'sex therapist' from the Chancellor's London home.
The cross-party committee did not find that Mr Lamont broke existing rules over the declaration of interests and said no further action was needed.
His interpretation of the rules - that he did not need to register because he could not be influenced by a payment of which he was 'completely ignorant of the source or sources' - was 'tenable' the MPs said.
But they indicated concern over Mr Lamont's actions by going on to tighten the rules, adding that the 'fundamental maxim' for any MP is 'if in doubt, register, or at least consult the registrar (of members' interests)'.
That was not done in Mr Lamont's case, they noted, and 'applies to anonymous benefits as much as to any other'.
The MPs' verdict came as the powerful Public Accounts Committee ensured continued scrutiny of the affair by calling Sir Robin Butler, the Cabinet Secretary, Sir Terence Burns, Permanent Secretary at the Treasury, and Sir Peter Middleton, his predecessor, to give evidence tomorrow.
They will be questioned over their judgement in providing a further pounds 4,700 - this time of taxpayers' money - to cover additional costs that Mr Lamont incurred by having private lawyers handle media inquiries about the letting.
Alan Beith, the Liberal Democrat's Treasury spokesman, said the Chancellor had been 'let off on a technicality' by the committee over his failure to declare the pounds 18,000.
The committee said that it 'recognises and to some extent shares' the concern expressed by Alice Mahon, the Labour MP who brought a complaint against the Chancellor, that if his view was accepted then 'any MP can avoid registering almost anything . . . by claiming ignorance of the source'.
Anonymous donations, the committee said, 'have the potential to be more compromising to members and more disquieting to the general public than gifts from identified sources', and should be declared.
It also wants any future payment from party sources declared 'because it would be difficult to argue that it did not arise in any way out of the recipient's membership of the House'.
Conservative Party sources deny that a special fund exists to help ministers or MPs who get into financial trouble.
However, they do confirm that MPs receive help from the party on a case-by-case basis over debt or other financial trouble. One Tory MP said: 'It doesn't happen once a week, but perhaps once a year.'
Close colleagues of Mr Lamont yesterday took the view that he could be 'reasonably relaxed' about the report, given the committee's finding that his interpretation was 'tenable'. Bob Dunn, a Conservative member of the committee, said that the Chancellor of the Exchequer had been in 'uncharted waters' and his case had shown a gap in the rules that needed to be filled.
But Bob Cryer, a Labour member of the committee, said there was 'explicit criticism' of the Chancellor for failing at least to consult and accused Mr Lamont of not declaring the payment 'because he wanted to avoid transparency and the fact that he had received pounds 18,000 from a Tory party slush fund'.
Mr Beith said: 'It is extremely difficult to believe that the Chancellor does not know, or does not want to know, where the money came from.'
The committee on members' interests also underlined that ministers are just as much subject to the rules over declaring interests as other MPs.
Select Committee on Members' Interests; First Report 1992-93; HMSO; pounds 3.50.Reuse content