Two chief constables will not allow their officers to march in uniform at a gay pride event, it has been revealed.
Over 100 officers are expected to march in uniform at the event
The Gay Police Association said chief constables in Hampshire and Dyfed-Powys were the only ones who refused.
The GPA said participation "improved" relations with the gay community. But Hampshire police said officers should not wear uniform socially.
More than 100 British police officers are expected to march in uniform at the Pride London rally on Saturday.
Ian Saunders, a GPA spokesman, said: "We write to senior officers about this every year. Some have given us standing permission.
"Both the chief constables [of Hampshire and Dyfed-Powys] had an issue with police officers wearing uniform off duty.
"What we would say is that this is an extremely worthwhile thing for officers to be doing on their day off, to put their uniform on and take part in something which raises community confidence."
In a statement, Hampshire Chief Constable Paul Kernaghan said: "I am very sympathetic to the objectives of the parade."
However, he added: "I have a very clear view that police officers should never wear uniform except while on duty.
"It is my view that participation in uniform could undermine the traditional neutrality of the police service."
"I would like to wish all who participate in the parade well. I trust it will be a positive celebration of diversity in action," he added.
His sentiments were echoed by Dyfed-Powys, which issued a statement that said: "It has long been our policy to refuse to allow officers to wear their police uniform when not engaged in operational duties outside the police force area."
However, it went on: "The force has discussed the issue with its own Gay and Lesbian group and the chief officer group has approved that any officer wishing to attend may do so, and may wear a T-shirt acknowledging their position within Dyfed Powys Police Force."
The event will be the sixth time uniformed police officers have taken part in gay pride parades in the UK and the third time in the capital.
Around 30,000 people are expected to attend the Pride London event.