Senior Liberal Democrats have received letters from the party’s jailed donor begging them to intervene over his treatment.

Sir Menzies Campbell, the leader, Charles Kennedy, the former leader, and Lord Rennard, the chief executive, have all turned down the request from Michael Brown.

The letters, seen by The Times, ask Brown’s former beneficiaries to contact the prison authorities and ask why he is being held in a high-security wing and has been taken off the prison job rota. The Lib Dems have refused to help, saying that they cannot intervene because of House of Commons convention.

The exchange will deepen the embarrassment of the party after the downfall of the financier, whose company — 5th Avenue Partners — donated £2.4 million in 2005, the biggest single donation received by the party. Brown was jailed for two years last September for perjury and a passport offence.

In the letters, sent last weekend from Elmley prison in Sheerness, Kent, Brown reminded the party leaders why they might wish to help him. “You may previously recall that I made a substantial donation to the Liberal Democrats,” he wrote.

“Unlike donors to other political parties, I have never sought any reward, honour or return for my financial support, nor would I, and feel very saddened at the resulting negative interest and perception of my involvement. However, I am pleading with you to help me as I believe that I am being treated very unfairly in prison.”

He then said that because he had authorised a press statement that appeared in this newspaper last Friday, he had been singled out for unfair treatment: “As a result of the latest coverage in The Times, I have just had my prison duties and internal privileges withdrawn and moved into a high-secure unit of the prison.”

The withdrawn privileges could include taking part in new inmate inductions and handing out food to fellow prisoners. Brown has also been placed in a secure block for Category B prisoners and is sharing a cell with two others.

At the end of the letters, Brown makes a final plea to his former political allies. “Given the climate of prison review and overcrowding in jail, I am one candidate who is ripe for tagging release,” he wrote.

Brown, 40, a Glasgow-born businessman with homes in Majorca and Mayfair, came to prominence in 2005 when he met Mr Kennedy, who was then the party leader. Even though he was not a party member and lived abroad, Brown liaised closely with senior Lib Dem figures.

As well as handing over a substantial donation, Brown gave Mr Kennedy a lift in a private aircraft to the party conference in Harrogate, North Yorkshire, and a return flight accompanied by Mr Kennedy’s wife, Sarah. The aircraft was also made available for the leader’s election campaign tour.

When Mr Kennedy’s son, Donald, was born in April 2005, Brown and his wife, Sharon, sent baby clothes, a hamper of food and a bouquet.

But Brown fell out with the Lib Dems after Mr Kennedy was kicked out of the leader’s job. He described Sir Menzies and other party figures as “muppets”.

The other recipient of Brown’s letter, Lord Rennard, was interviewed as a witness last autumn by detectives trying to trace millions of pounds alleged to have gone through Brown’s company. Brown denies any wrongdoing.

The peer helped to oversee the donation when it was received in four tranches between February and March 2005.

A Liberal Democrat spokesman said that the three recipients of the letter would not help. Lord Rennard has replied, suggesting that Brown ask his solicitor to intervene.

Lord Rennard wrote: “There is a convention among MPs that they don’t take up individual cases on behalf of constituents other than their own. Therefore neither Charles Kennedy nor Menzies Campbell is able to act on your behalf.”

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