The letter’s signatories are 103 business leaders, including senior executives from FTSE 100 companies and small and medium sized businesses and entrepreneurs.
It said the Conservative-led government "has been good for business and has pursued policies which have supported investment and job creation".
The letter backed the policy of reducing corporation tax to 20 per cent, which it said has helped Britain grow "faster than any other major economy last year".
It added: "We believe a change in course will threaten jobs and deter investment. This would send a negative message about Britain and put the recovery at risk."
Murphy has been chairman of the 147-store garden centre chain since 2012.