Ed Miliband claims his campaign adviser made a "slip of the tongue" when she suggested the Labour leader may break the promises he had carved into an 8ft stone.
Eyebrows were raised when Mr Miliband unveiled a limestone slab inscribed with six pledges and claimed it could be installed in the Downing Street garden.
The #EdStone hashtag trended on Twitter.
Asked about it on BBC 5 Live, Lucy Powell said: "I don't think anyone is suggesting that the fact that he's carved them into stone means that he is absolutely not going to break them or anything like that."
The vice-chair of Labour's campaign went on to say Mr Miliband "stands by his promises".
Opponents jumped on the comments - but in an interview with Sky News Radio the Labour leader reiterated his commitment to the six pledges.
"It was a slip of the tongue," he said.
"Look, we're very clear: I'm going to deliver my pledges, that's why they're carved in stone. They're not going to expire on 8 May like the other parties' promises."
On Ms Powell, he added: "She's brilliant, she's played a brilliant role in this campaign. She's done an extraordinary job and we all make slips of the tongue."
Conservative Party chairman Grant Shapps said Ms Powell's comment was proof "Ed Miliband has no intention of keeping the promises" because "his sums simply don't add up".
He added: "£30,000 worth of limestone doesn't change the fact that Ed Miliband will say one thing and do another."
In later comments on Twitter, Ms Powell said: "Honestly, Tories and others desperately misquoting what I said. Anyone who heard the whole interview knows I said the opposite."