Dr Campbell has received a CBE for services to science
Political expediency "rules the roost" over scientific advice, one of the three latest government drug advisers to resign has told BBC News.
Dr Simon Campbell quit the Advisory Council for the Misuse of Drugs on Monday, over the sacking of chief drugs adviser Professor David Nutt.
Alan Johnson dismissed Prof Nutt for "crossing a line" into politics.
Dr Campbell said home secretaries would listen to scientific advice but had usually already made their decisions.
He told BBC Radio 4's Today programme: "When we made our recommendation on cannabis we saw no reason to change the classification and yet the government has already decided to move from Class C to Class B."
"That can only be because the government saw it as a votes-catching exercise."
But he said giving advice did not feel like a waste of time, because it was such a vital issue for the nation.
"The misuse of drugs is incredibly important, which is why as unpaid volunteers, we're prepared to put our time in and hopefully the government will take notice," he said.
Explaining why he had resigned, he said: "I do not accept the reasons given for Professor Nutt's dismissal, that was a clash of personalities.
"I do not agree with the manner in which he was dismissed. Such an abrupt dismissal was a humiliation for such a respected scientist."
On Monday, members of the council had met Alan Johnson seeking reassurance their independence would not be compromised.
Following the meeting, Dr Campbell, Dr John Marsden, and pharmaceutical consultant Ian Ragan resigned.
Dr Campbell said his resignation had been announced to the media as he had been travelling home from the meeting.
"I sent a formal letter to the home secretary late last night, but I'm dismayed my resignation was on the BBC news as soon as I got home."
Dr Campbell is a synthetic organic chemist who received a CBE for services to science.
Five members of the council have now resigned following Prof Nutt's dismissal.
BBC home affairs editor Mark Easton said their absence meant the council was "stymied" and was effectively now unable to operate under its current constitution.
He added the remaining 25 members still had outstanding concerns and Mr Johnson had been presented with "a big problem".
The Home Office confirmed the ACMD had to have 20 members to remain quorate, and that six positions had to be filled for the advisory group to function.
Prof Nutt was sacked after calling cannabis less harmful than alcohol and nicotine, and saying it had been upgraded to Class B for political reasons.
Mr Johnson said Prof Nutt had "crossed the line between offering advice and then campaigning against the government on political decisions".