Sinn Féin have been accused of wanting to put more soldiers and security personnel from the Troubles in the dock, by the DUP.
Party MP Nigel Dodds said on Tuesday in the House of Commons that Sinn Féin had collapsed the Stormont government as part of a political agenda.
"We share the deep regret at the highly irresponsible decision of Sinn Fein to single-handedly cause the collapse of the present executive and precipitate what the secretary of state has rightly called this threat to the continuity of the devolved institutions,” said Mr Dodds.
"It's clear from what Sinn Fein have said in their resignation letter, it's not about RHI, because had this continued we would have had the investigation and proposals to mitigate cost.
"But it's because, according to them, they're not getting their own way in a whole series of demands, including rewriting the past and putting more soldiers and security forces in the dock - despite the fact they've just agreed a programme for government in Northern Ireland."
Conservative MP Richard Benyon, who served in Northern Ireland, said the investigations were "a very one-sided judicial process".
Mr Lewis said: "He has ably expressed the dismay at the grotesquely partisan and inequitable decision to instruct the Police Service of Northern Ireland to start pursuing retired British service personnel, whilst amnestied former terrorists freely walk the streets.
"Will the Government bring forward legislation urgently, to offer them at least the same protection as the amnestied terrorists undeservedly enjoy?"
Northern Ireland Secretary James Brokenshire said there were no amnesties that amounted to immunity from prosecution.
He added: "He makes a broader point in relation to the need for a proportionate, balanced approach to legacy, to ensure that all aspects are investigated properly, rather than looking at one side rather than the other side.
"That is precisely the approach I believe can be taken forward through the Stormont House agreement."