The British Conservative Party has been accused of engaging in sectarianism over its secret talks with the UUP and the DUP.
Representatives of the parties met most recently on a possible unity move at an English stately home outside of London on the 16th and 17th of January.
Dr McDonnell has sought an urgent meeting with Conservative leader David Cameron to express his concerns.
The South Belfast MP said the meeting showed contempt for nationalists and adds further mistrust to the political process.
“It’s saying to me that the Conservatives at this stage are not fit for government,” said Dr McDonnell.
“This is playing the orange card, it’s being sectarian, it’s being divisive and I will be putting it to David Cameron that it was naive at best and malevolent at worst.”
In 2008, the Conservative party officially joined forces with the Ulster Unionist Party, forming a new electoral alliance known as the Ulster Conservative and Unionists - New Force (UCUNF).
But at the weekend, UUP leader Reg Empey insisted the pact only covered European and Westminister elections and gave a strong indication that his party was exploring the possibility of a link-up with the Democratic Unionist Party at the Stormont Assembly in Belfast.
He confirmed the UUP were involved in a serious engagement about cooperating politically with the DUP and said nothing had been ruled in or out.
Ian Paisley Jnr od the DUP said nationalist concerns about a secret unionist strategy were “a smokescreen to cover the anger that certain nationalists will have when they realise their political careers are over because they were based on the false dawn of a split unionist vote.”
Mr McDonnell described the move as “cheap Tory stroke”.
“For Mr Cameron to orchestrate playing the orange card last weekend for an hour of self interest did our process serious damage,” he said.
“It disrupted the discussions of devolving policing and justice at a very delicate point just to let him gain some narrow political advantage in the event of a hung parliament.”
Dr McDonnell also accused Mr Cameron of exploiting the crisis at Stormont.
“No-one is buying the Tory line that this secret, all-unionist meeting was an attempt to overcome political instabilities,” he said.
“If this was the genuine motivation then why haven’t the Tories met with the nationalist parties which represent half of the population living here?”
Leader of the moderate unionist Alliance Party leader David Ford told his annual conference that any talk of unionist non-sectarian politics will be “sunk without trace” if a pan-unionist front is agreed.
That was demonstrated by the weekend resignation of three local Conservatives, two of whom are Catholics, who had been nominated to stand in the Westminster elections, he said.
“Tories in Northern Ireland have claimed to be non-sectarian and progressive, saying that they are seeking to introduce what they describe as ‘national politics’ to our society,” he told delegates.
“However, if there is any truth whatsoever to the talk of unionist pacts and realignments with the DUP also included, the claims of non-sectarian progressive politics are sunk without trace,” he said.
“If there are members within the ranks of the local Conservatives who genuinely believe in a shared future as a priority, I fear they are now in an impossible position,” added Mr Ford.