Also re-appointed, the Foreign Secretary Boris Johnson is said to be on manoeuvres and sounding out MPs about a leadership bid.
"But what I'm doing now is actually getting on with the immediate job". I think she should probably resign and there should probably be another general election.
Many MPs are angry over what they see as an unnecessary vote that has cost several lawmakers their seats and are demanding she run a more open, collegiate government after her first months of a dictatorial regime.
Mr McDonnell said Labour should "draw on upon all the talents" when it considered its new frontbench although he also gave praise to the existing shadow cabinet.
The pound fell sharply after Britain's election saw the Conservatives lose their majority in parliament, raising questions about the next government's ability to lead the talks to leave the European Union. Meanwhile, the Labour Party started the campaign looking as if it was maybe facing some sort of existential disaster.
The strength of any deal looks set to be tested when the Commons meets, with Jeremy Corbyn vowing to try to bring down the Government by defeating Mrs May in Parliament and insisting: "I can still be prime minister".
With both of her chiefs of staff leaving Downing Street it is clear she is willing to compromise with discontented MPs and to learn some of the lessons of a disastrous election campaign.
The decision comes as the Prime Minister has announced she's keeping her core cabinet members in place. If not, the opposition Labour Party would expect to have an opportunity to put forward an alternative Queen's Speech and see if it could win the support of a majority in parliament.
"I am going to be backing her, and absolutely everybody I'm talking to is going to be backing her too", said Johnson, who had been touted as a possible successor to May.
'The people of Britain have had a bellyful of promises and politicking.
The lack of a decisive victory for the Conservatives also called into question how the United Kingdom will negotiate its exit from the European Union. But the ballot-box humiliation has seriously - and possibly mortally - wounded her leadership just as Britain is about to begin complex exit talks with the European Union.
Former Conservative finance minister George Osborne, who Mrs May sacked after taking office, said she was now a "dead woman walking". May has said her government will go ahead with these discussions as planned.
Defence Secretary Michael Fallon said on Sunday that the plans were unchanged but added it was "very, very important that we're careful about the existing trade that we do with Europe, about access to the single market".
Despite her party losing 12 seats, May has not resigned and has asked the Queen's permission to form a minority government with the support of the Democratic Unionist Party (DUP) from Northern Ireland, who won 10 seats on Thursday night.
She is seeking a so-called confidence and supply deal, which would involve the DUP supporting the Conservatives on key votes but not joining a formal coalition.
"If ... the Conservative Party has won the most seats and probably the most votes then it will be incumbent on us to ensure that we have that period of stability and that is exactly what we will do".
May spoke to the Irish Taoiseach (Prime Minister) Enda Kenny over the phone on Sunday.
They also hold a number of controversial views - as well as opposing the legalisation of abortion, the party have a strong anti-LGBT record and have continuously come out against same-sex marriage.
Dr Fergal Davis, Reader in Law, King's College London, is a constitutional law expert who has been observing and writing about Northern Ireland's law and politics for 20 years.