Brexit campaigners are being held to account for claims made that an extra £350 million a week could be spent on Britain's health system if the country left the European Union.
Lobby group Open Britain wants to ensure the UK remains "open and inclusive" with a positive relationship with Europe following the Brexit vote.
It's called on opposition group, Change Britain, to honour the pledge of £350 million a week ($630 million) that could be spent on Britain's National Health Service if Britain was outside of the EU that was plastered on a bus and became a key mantra of the Vote Leave campaign.
Immediately after the vote, Brexiteers appeared to back away from their pledge with UKIP leader Nigel Farage calling it a "mistake".
The UK's actual contribution to the EU budget, once rebates are taken into account is around £120 million a week or 26 pence per person per day, according to fact checking site Infacts.
A new campaign by Change Britain voiced by Foreign Secretary Boris Johnson, is designed to unite the UK and help make a success of the decision.
However many are angry this core pledge remains missing from their site. An open letter from a group of Labour MPs argues "there should be no escaping this pledge" and calls on the Brexiteers to honour it.
"We warned this was a lie - and so it turns out to be," the letter states.
"But there should be no escaping this pledge for the Leave campaign - they cannot walk away from it now, disown it or pretend it never happened. They must either admit it was a lie and apologise to their voters, or justify it and explain when it is coming.
"On behalf of the communities we represent, we will keep asking, where is the £350m for the NHS that was promised by the Leave campaign? Many will have voted to Leave based on this pledge. Those who made the pledge must now be held to account for the promise they made."
Since the June 23 vote the UK government has been scrambling to formulate a plan for what the relationship with the EU will look like.
Prime Minister Theresa May has said "Brexit means Brexit" and ruled out a second referendum, however there has been increased scrutiny over terms of the debate and whether voters had enough information about the choice they were faced with.
Conservative MP Anna Soubry, who was a pro-Remain campaigner, said Vote Leave should "should hang their heads in shame".
"There were many people, particularly in less wealthy areas, who were convinced by Leave's claim that if we left the EU we would be able to pour millions more into the NHS," she said.
"The danger now is these people will become even more disillusioned with all politicians because this lot misled them."
Change Britain has not yet commented on the omission. However Vote Leave campaigner Gisela Stuart said the £350 million claim was just an "example" that should not have been taken literally by voters.
"The NHS was the example of if you're spending that amount of money and you haven't got control of what to do with it, I would spend it on the NHS. It was about taking control of your spending decisions," she said.