Theresa May could risk the peace process in Northern Ireland over concerns she is pandering to the Democratic Unionist Party so they will back her Brexit plans, a leading Northern Irish politician has warned.

Naomi Long, who is leader of the anti-sectarian Alliance Party, told The Independent there were growing concerns in Northern Ireland that the Prime Minister’s impartiality on the peace process is being compromised by a need to keep the DUP onside.

The DUP has eight MPs at Westminster, which could prove essential support for the Conservatives who currently have a slim majority in the House of Commons.

Amid growing concerns that some pro-EU MPs could rebel against Ms May, securing support from the DUP is being seen as increasingly important in order to deliver her plans for the UK's withdrawal from the EU.

On Monday, Northern Ireland Deputy First Minister Martin McGuinness resigned, effectively ending power-sharing at Stormont. He cited concerns over the DUP’s “arrogance” in how it has handled allegations of a major financial scandal, known as the “cash for ash” affair.

His DUP counterpart Arlene Foster is accused of mishandling a Government project on renewable energy, which has cost the taxpayer more than £400m.

Speaking to The Independent, Ms Long said: “There is a growing perception in Northern Ireland that the potential usefulness of DUP votes in Westminster to advance Brexit may be compromising the UK Government’s willingness to challenge the DUP and ability to act as honest broker and impartial guardians of the Good Friday Agreement.”

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She warned failure to do so could result in “the permanent collapse of the political institutions”.

Ms Long, who represents the East Belfast constituency, revealed she had written to Ms May’s Government on two occasions last month, warning of Stormont’s imminent collapse and urging action.

She says the Government failed to act, which fuelled turmoil in Northern Ireland and led to Mr McGuinness’ resignation.

The letters, seen by The Independent, urge the Government to launch an inquiry into the “cash for ash” financial scandal, warning “failure of the executive to properly address this issue could have very significant consequences [which could] present a serious threat to the stability of the devolved institutions in Northern Ireland [and] could trigger the collapse of the executive”.

Ms Long said the Government failed to take her warnings seriously and did not act. She told The Independent: “It has been clear that the current government, led by Theresa May, is completely tone deaf to the issues in Northern Ireland with neither the experience nor frankly the interest to recognise the political sensitivities.”

The “cash for ash” scandal allegations relate to the Renewable Heat Initiative which was designed in 2012 to encourage local businesses to use renewable energy sources. However, the scheme appears to have had serious flaws, resulting in a loophole which meant businesses were given a financial incentive to burn fuel pointlessly.

Ms Foster, who became First Minister last year, was the minister in charge of the scheme at the time and has subsequently come under considerable criticism. The allegations emerged via a whistle-blower in November and intensified in December when one of Ms Foster’s party colleagues, Jonathan Bell, alleged she had asked civil servants to alter documents to reduce the appearance of her role in the affair.

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She denies any wrongdoing and says she has been unfairly represented in the media. She resisted calls to resign and survived an attempted vote of no confidence at Stormont.

Under power-sharing rules, Mr McGuinness’ resignation means Ms Foster also loses her position as both must participate equally in government.

Secretary of State for Northern Ireland James Brokenshire now has a week to announce Northern Ireland will face new elections.

A spokesperson for the Northern Ireland Office told The Independent: “This is complete nonsense. Since the issue emerged, the Secretary of State has been closely engaged with leaders of all NI political parties in a bid to ease tensions and to find a way forward.

“Both the Secretary of State and the Prime Minister have made clear that this Government is firmly and fully committed to the Belfast Agreement and its successors and will continue to uphold these, and the principles they set out, in full.