Senior Conservative officials have accused West Midlands police of failing to do enough to keep protesters away from delegates at the party conference, a leaked letter reveals. The disclosure comes just hours after the police were forced to lock down the conference in central Birmingham for several hours after a security scare. Police say they have been shortchanged by over £500,000 on the costs of keeping the conference in Birmingham secure. Jake Berry, the Conservative party chair, and Darren Mott, party chief executive, sent a letter on Sunday evening to the police and crime commissioner for the West Midlands raising “serious concerns” over security around the conference. “We are writing to raise our serious concerns at the measures taken to protect delegates attending the 2022 Conservative party conference,” they wrote. “Over the course of the last 24 hours, we have received reports from some of our delegates that they have faced intimidation and threats of violence from some outside the venue.” The letter, seen by the Guardian, continues: “It of course goes without saying that we fully support the rights of all peaceful protesters, but with reports of delegates being spat at, chased and in some cases physically assaulted as they enter and exit the conference centre – this is clearly not acceptable.” In a reply, Simon Foster, the police and crime commissioner, said: “It is certainly the case that there have been more protesters this year than at previous conferences, and these protests have been more volatile than previously. This is doubtless a reflection of the heightened national political tensions. “Chief constable Sir David Thompson and assistant chief constable Jane Meir have briefed policing minister Jeremy Quinn and West Midlands mayor Andy Street over the weekend, and provided assurances concerning the safety of delegates and policing of protests. “I can report that, to date, no offences have been recorded, and the issues raised have not reached the criminal threshold. No arrests have had to be made to date.” Foster complained about police being shortchanged over the costs of keeping the conference secure. Dozens of officers have been brought from other forces to help patrol the canals and roads of the city. “The Home Office has withdrawn the previous flexibility to claim costs associated with paying officers for rest days worked during the conference,” he said. “I cannot see any benefits or efficiencies arising from this change. “It means that where officers work rest days, instead of having the option to be paid for them, they will have to take the accrued rest days in the subsequent 12 months. The adverse impact on the operational capacity of West Midlands police and the consequential impact on the people and communities of the West Midlands will be significant. “The change also means the operation is more reliant on mutual aid from officers outside of the force. “In addition, it is my understanding that the change will increase the net overall cost to the taxpayer associated with the conference, and increase the net cost to West Midlands police. “It was already the case that we could claim only a portion of the costs associated with planning for the conference – imposing a net cost on West Midlands police of over £500,000 – but the changed policy on rest days could add hundreds of thousands of pounds more to the costs that land on West Midlands police,” Foster wrote. Since the conference began at Sunday lunchtime, ministers including Jacob Rees-Mogg have been chased and heckled by protesters shouting “Tory scum”, with one woman seen running towards him shouting “You have family too” and blowing an air horn in his face. Protesters, including the anti-Brexit campaigner Steve Bray, have also played loud music including the Benny Hill and Muppets theme tunes in an effort to mock the Tories as the chancellor, Kwasi Kwarteng, was forced into a humiliating volte-face over the abolition of the 45p tax rate. The Birmingham International convention centre went into “lockdown” at about 3.30pm on Monday after a security alert, leaving many MPs stranded outside. The security announcement came just minutes before Kwarteng was due to make a highly anticipated speech to members.