The shadow chancellor says Labour MPs will not be “bribed” into supporting Theresa May’s Brexit deal in exchange for funding for their constituencies.
John McDonnell said any such offer would be “pork-barrel politics” and “dangerous for our democracy”.
It follows reports that investment could be made in Leave-voting constituencies to secure MPs’ votes.
A spokesman for the PM said this week any investment to tackle inequality could not be called “cash for votes”.
The government is understood to be considering proposals from a group of Labour MPs in predominantly Leave-supporting constituencies to allocate more funds to their communities for big infrastructure projects.
It is thought the MPs have urged the prime minister to consider re-allocating the EU’s regional aid budget away from big cities and local councils and to give the cash direct to smaller communities, often in former steel and coal mining areas.
On Thursday, Labour MP John Mann, who was one of only three Labour MPs to back Mrs May’s Brexit deal, told her to “show us the money”.
On a visit to Stoke-on-Trent on Saturday, however, Mr McDonnell rejected any link between votes and funding.
He said: “I can’t see individual MPs, to be honest, selling their vote in parliament in this way and what most MPs I’ve spoken to are saying is if there is money to invest it should be invested anyway so just get on with it.”
“Don’t expect us to be bought and bribed in this way – that’s a form of corrupt politics that we don’t want to be introduced into our political system.”
Mr McDonnell also criticised the government over its “supply and confidence” arrangement with the Democratic Unionist Party (DUP).
The DUP negotiated a deal for an extra £1bn in spending for Northern Ireland over two years in exchange for supporting Mrs May’s minority Conservative government, following the snap election in 2017.
Mr McDonnell said: “It was something like £100m a vote they spent to get the DUP supporting them, so they already introduced that pork-barrel contractual politics.
“I think it degrades our political system and to try and extend it in this way, I think it’s dangerous for our democracy.”
His comments follow Labour Party chairman Ian Lavery saying that accepting any offer of funding “would be fool’s gold”.
Mr Mann, MP for Bassetlaw, a former coal mining area in Nottinghamshire, met cabinet office officials in Whitehall earlier this week.
Speaking afterwards, he told reporters: “I want to see, when we leave the European Union, significant investment in new technologies, new jobs, science and industry in areas like mine and all the other areas in the country like mine.
“This isn’t transactional politics, this is about getting a national fund… the areas that voted Leave the most are the areas that have not had that investment.”
Ministers are continuing to try to win support for the withdrawal deal Mrs May has negotiated with the EU, which was rejected by a historic margin in a Commons vote more than two weeks ago.
A spokesman for the prime minister this week confirmed ministers were looking at a programme of “national renewal” following Brexit to tackle inequality and rebuild communities, but that he “absolutely wouldn’t characterise” the reported investment offer as “cash for votes”.
This week, a backbench amendment to replace the Irish backstop with “alternative arrangements” in the deal, won the support of the Commons.
The PM says she will now return to Brussels in a bid to re-open negotiations.