The investigation centred on claims that rules governing elections had been broken by a newsletter published by British Nuclear Fuels Ltd, apparently promoting Dr Cunningham's candidature at the start of the campaign.
BNFL insisted the timing of the four-page newsletter, featuring an interview with the MP supporting controversial plans to build a pounds 2bn underground store for nuclear waste, was accidental.
It was published on 12 March - the day after the general election was announced - but printed days in advance. Sellafield, with 13,000 workers, is the area's biggest employer.
The allegation was made by Philip Davies, the Tory candidate for the Copeland seat. Mr Davies's complaint was lodged after he met a senior legal adviser at Conservative Central Office, and Penningtons, the party's solicitors.
A Cumbria Police spokesman said yesterday that the Director of Public Prosecutions had studied the evidence gathered by an inquiry team and ruled there had not been a breach of the Act.
Dr Cunningham said yesterday: 'I always believed that the allegation was absurd.'
Cumbria Police are continuing to investigate a further claim by Copeland Conservatives that the cost of a glossy brochure celebrating Dr Cunningham's 21 years as Copeland MP should have been included in his election expenses.