Sir Jeffrey Donaldson: Resignation of DUP leader poses serious challenges across party says Prof Jon Tonge

The author of the book, The Democratic Unionist Party: From Protest to Power, Prof Tonge described the news as "a hugely dramatic development".

He added: "The unrivalled leader of unionism has fallen and it is very difficult to replace him. [former Deputy Leader] Gavin Robinson will have to grow very quickly into the job. It is not being disrespectful to Gavin to say he wouldn't have the stature of Jeffrey Donaldson. You have to remember Jeffrey Donaldson is a veteran parliamentarian first elected to Westminster in 1997."

The recent deal agreed by the Government and DUP which secured the return of the Assembly has been called "The Donaldson deal" because he was the person selling it to a sceptical audience, he said.

So further down the track if some new contentious EU law was applied to Northern Ireland and Sir Jeffrey is not there to shore the deal up, he sees “trouble” because some MLAs and most MPs in the party oppose it.

Prof Tonge also queries whether the position of Deputy First Minister Emma Little Pengelly, who was nominated for the job by Sir Jeffrey, will now be reviewed.

"I don't see the institutions collapsing anytime soon. But I do think in the longer term, unless Gavin Robinson can get to a level Jeffrey Donaldson was at - which is probably asking too much - I see difficulties ahead for the party."

"I think a Lagan Valley by-election is more likely than not, although Jeffrey Donaldson, given that he is contesting the charges vigorously, may choose to fight on as an independent MP."

Until now Prof Tongue did not think Alliance’s Lagan Valley MLA Sorcha Eastwood had a chance of taking the seat, but now he thinks a by-election would be "very close indeed".

"If there's not a by-election then, the general election is impossible to predict because we don't know whether the legal case will be resolved by then."

However he also sees a risk to the new DUP leader Gavin Robinson, because he does see his east Belfast seat guaranteed in the forthcoming general election.

"I think politically, the DUP is damaged because Jeffrey Donaldson was the DUP's most visible leader. He was a popular person across political divides, and he was popular among the bulk of the DUP's base so they've lost their biggest figure, and that's a tough one for any political party to take. Because whilst if acquitted of these charges he could make a political comeback, in the short term whilst due legal process is applied, he's out of the political game."

There's now a risk of a three way split in unionism, he believes.

"Unionism demographically does not have the numbers to have a proper three way split between the DUP, UUP and TUV. Unionism is stronger when it has got one big party. There is a risk the TUV will be snapping at the DUP's heels, probably even more now."