Former US president Bill Clinton has backed Northern Ireland politicians in their claims to have struck a deal with London over central government funding, Deputy First Minister Martin McGuinness said.
Mr McGuinness told the Stormont Assembly that the Labour government made commitments in the St Andrews political deal of 2006 that paved the way for the power-sharing government.
He said the current Government is duty-bound to uphold those commitments and said he got the support of Mr Clinton in private discussions.
Last week Deputy Prime Minister Nick Clegg used a trip to Northern Ireland to claim that in top-level talks in London, Stormont leaders failed to raise the pledge of £18 billion to modernise the Northern Ireland infrastructure.
But Mr McGuinness claimed the issue topped the agenda in a meeting with the Chancellor and he claimed to be surprised by Mr Clegg's comments.
"The reality is and the fact is, and the minutes will show, that this issue was raised very forcibly in the course of that meeting," said Mr McGuinness.
"I think that it clearly is a worrying development that you have a Deputy Prime Minister who wouldn't be aware of the reality of that engagement.
"It was also very significant that at that engagement the Chancellor George Osborne asked the First Minister (Peter Robinson) and I to furnish him with the details of the agreements made at St Andrews and the discussions which followed."
Mr McGuinness said the pledge had been made public by Gordon Brown in a statement outside Downing Street at the time, while Mr Brown's "guarantee" was recorded in documents lodged at Westminster.
Mr McGuinness said: "There can be no doubt about it - the issue was raised and has been raised ever since."