A senior Liberal Democrat minister will launch a stinging attack on Theresa May, the Home Secretary, as he announces his resignation from the Government tomorrow.

Norman Baker, the crime prevention minister, is stepping down after a year of internal battles within the Home Office with his Conservative boss.

In a scathing verdict on Ms May’s leadership, Mr Baker warned that support for “rational evidence-based policy” was in short supply at the top of her department.

The Lib Dem has publicly clashed with Ms May on issues including drugs policy and immigration.

He told The Independent yesterday that the experience of working at the Home Office had been like “walking through mud” as he found his plans thwarted by the Home Secretary and her advisers.

“They have looked upon it as a Conservative department in a Conservative government, whereas in my view it’s a Coalition department in a Coalition government,” he said.

In a scathing verdict on Theresa May’s leadership, Norman Baker warned that support for 'rational evidence-based policy' was in short supply at the top of her department (PA)

“That mindset has framed things, which means I have had to work very much harder to get things done even where they are what the Home Secretary agrees with and where it has been helpful for the Government and the department.

“There comes a point when you don’t want to carry on walking through mud and you want to release yourself from that.”

His resignation is further evidence that relations are rapidly deteriorating within the Coalition ahead of next year’s general election.

It follows an acrimonious clash between Mr Baker and senior Conservatives over a three-month delay in publishing a Home Office report on drugs policies abroad. He accused them of holding it up because its findings could be politically embarrassing to the Tories.

In his resignation letter to the party leader Nick Clegg, Mr Baker said: “I regret that in the Home Office, the goodwill to work collegiately to take forward rational evidence-based policy has been in somewhat short supply.”

Mr Clegg, who tried to persuade him to change his mind, paid tribute in reply to a “brilliant” minister who has always been “determined to deliver a more liberal agenda for Britain”.

Mr Baker, the MP for Lewes, had been an unexpected appointment to the Coalition Government after the election in 2010.

His switch to the Home Office in September 2013, with instructions from Mr Clegg to raise the Lib Dems’ profile in the department, caused even greater surprise. Yesterday he said he was proud of his part in tackling female genital mutilation, promoting alternatives to animal experiments, bringing in a new approach to combating anti-social behaviour and championing an “evidence-based” approach to drugs policy.

But he said Ms May had found it “hard to accept” that he had been given the role by Mr Clegg of ranging across Home Office policies. “To be fair to the Home Secretary, I think she takes the view she has to keep a tight leash on everything otherwise she wouldn’t carry on as Home Secretary – she has been there four-and-a-half years.

“I think she is quite competent and professional, and I have a lot of respect for her professionally,” he said.

“I just think it’s a pity that she took the mindset that the Lib Dems had to be put up with, and we were almost a cuckoo in the nest rather than part of government.”

Discussing his very public row with Ms May on UK drugs laws, which he wants liberalised, Mr Baker added: “The phrase ‘soft on drugs’ is used. What is soft on drugs? I think being soft on drugs is pretending there isn’t a problem, and this is being sorted.

“If anyone is soft on drugs it’s my Conservative colleagues, because they are the ones who allow the process to go on whereby drug dealers continue to make money and people continue to get fined and carry on taking drugs.”

Lib Dems: policy focus Show all 6 1 /6 Lib Dems: policy focus Lib Dems: policy focus GEOFF PAYNE English Party representative, Federal Policy Committee “The Tories have been seen over the last week for the nasty party that they are. I would go on the offensive to justify our record and ruthlessly exploit those differences over things like the benefit cap and scrapping the Human Rights Act.” Lib Dems: policy focus GARETH EPPS Co-chair, Social Liberal Forum “We need to make it clear that we’re not in it for our mates, like the Tories. Some of the successes, like shared parental leave, would never have been there with only the Tories.” Lib Dems: policy focus DAISY COOPER Candidate, party president “A lot of people do know that raising the tax threshold is a Liberal Democrat policy. We just need to keep telling the public that.” Lib Dems: policy focus JOHN PUGH Southport MP, rebelled against leadership on tuition fees “We need to say that issues of social inequality and justice are equal priorities to economics. We must return to a traditional funding of the NHS.” Getty Lib Dems: policy focus LORELY BURT Solihull MP, former deputy leadership candidate “It does seem that we’ve been the repository of all the best ideas and that nice Mr Cameron has pinched our income tax policy and Labour the mansion tax policy.” Rex Lib Dems: policy focus BARONESS PARMINTER Former chief executive, Campaign for the Protection of Rural England “We have got to be very clear where we have stopped the Tories implementing changes that would have been detrimental to the environment.” LNP

He contrasted Ms May’s approach with the three-and-a-half years he spent as a minister at the Department of Transport under Tory secretaries of state.

He said he knew the Home Office would be “difficult in coalition terms” as it deals with issues such as immigration which the “Tories get rather obsessed about”.

But he added: “I suppose what I expected was a more collegiate response, so even if you disagreed about something we would have a proper discussion about it.”

Mr Baker said he would now spend time in his constituency, where he is defending a majority of 7,647, and with his family who have “paid a price” for his punishing workload.

He added: “I just need a break to get some perspective and recharge myself.”

Mr Baker, 57, who is married with a daughter and two step-daughters, is a keen amateur musician.