Ed Miliband's former director of communications has defended the much criticised changes made to the Labour Party's leadership election rules.
Tom Baldwin said supporters of Andy Burnham, Yvette Cooper and Liz Kendall should have spent less time "whinging" and more time signing up new members.
He told the BBC's Daily Politics programme on Tuesday that while "lots of mistakes were made" by Miliband and his team, the change to the leadership rules "was not one of them".
The defence came as The Huffington Post revealed a Tory minister had been allowed to remain a member of the Labour Party despite being given a job by David Cameron in his government.
Labour members, including former home secretary David Blunkett, have also complained about not receiving their ballot papers even though the new leader will be announced on Saturday.
Labour leadership elections used to be run on a three-way electoral college of MPs, members and trade unions. However Miliband changed the rules to a one member one vote system. He also introduced the "registered supporter" system which allows non-members to pay just £3 to vote in the leadership race.
The new system has led to complaints from many Labour MPs that the contest is being infiltrated by he hard-left, as well as Conservatives, who do not really support the Labour Party and want to see Jeremy Corbyn elected leader.
However Baldwin said it was "disgraceful" for moderate Labour MPs to attack the rules rather than focus on beating Corbyn.
"If you are losing an election, be it a leadership election or general election, the solution is not to complain about too many people voting, go out and win some more votes," he said.
"I am astonished that so many people in the Labour Party have spent time whinging about the rules rather than doing what Jeremy Corbyn's campaign was doing, which was making the rules work for them.
"Modernisers and mainstream Labour Party supporters should begin to work within the system to recognises they are many, the far-left are few, and sign up more people so next time they win."
The former Miliband aide also noted the changes introduced by Miliband had been supported by Tony Blair and was designed to "open up the Labour Party".
Baldwin also had a tense exchange with Labour MP Simon Danczuk, who branded Miliband "one of the worst Labour Party leaders in history" given the party's general election defeat.