MP Tom Brake said the budgetary commitments after Brexit include EU projects and pension contributions.
Speaking on her TalkRadio show, Ms Harley-Brewer said: “We haven’t got £40 billion worth of pension contributions. I mean, look at the Erasmus programme again – this is a student exchange programme – it’s £112 million a year. That’s not 40 billion quid, is it?
“There’s just no way you can get to the 40 billion number unless you’re offering just a goodwill ‘here you go, here’s some money’.
“I doubt very much that that is the way that the UK Government are negotiating. I doubt very much there’s a line on their spreadsheet that says ‘goodwill gesture – we’ll give them £10 billion as a goodwill gesture’.”
The MP for Carshalton and Wallington was not able to explain where the 40 billion sum comes from and blamed it on Government secrecy.
Mr Brake said: “What the UK is negotiating, presumably, is continued involvement in the number of programmes and may be seeking… some preferred access to the EU markets in the future in exchange for a contribution.
“But we don’t know that because our Government are not particularly open about what it is they are trying to negotiate.
Ms Hartley-Brewer hit back and said the Canadians and Japanese do not have to pay for tariff-free access to the European Union so there is no reason why Britain should pay for access.
Theresa May gained cabinet backing on Monday night to increase the financial settlement to a figure expected to be around €40bn.
But Mr Brake said the cabinet meeting will result in Brits having to recognise there is a price to pay to leave the bloc.
He said: “People like Boris Johnson and Michael Gove – who, during the referendum campaign, never said the UK might have to pay up to £40bn to leave the EU.”
“The UK, through the different negotiations we’ve been party to in recent years, has committed to collectively funding certain projects having made contractual agreements.”