The Holyrood by-election caused by the resignation of Labour MSP for Glasgow Cathcart, Mike Watson, will be held on Thursday 29 September.
Mike Watson resigned after pleading guilty in court
The vote will take place on the same day as the Westminster by-election in Livingston, sparked by the death of Labour MP Robin Cook.
Mr Watson resigned from his Scottish Parliament seat after he admitted wilful fire-raising last week.
Nationalists said the early date in Cathcart showed Labour in a panic.
Decisions about Scottish Parliament by-election dates lie with Holyrood Presiding Officer George Reid who took soundings from the main political parties.
Labour pushed for both contests to be held on the same day while Nationalists and Tories had argued that this would leave far too little time to prepare for a by-election in Cathcart.
A Holyrood spokeswoman said: "After the resignation of Mike Watson, the presiding officer has decided on 29 September in order that the people of Cathcart can be democratically represented in the Scottish Parliament at the earliest opportunity.
"The presiding officer wished to make sure that parties could go through the democratic process of nominating and selecting candidates in time for the election.
"He is satisfied that, while tight, this can be done."
The spokeswoman said that the presiding officer was concerned that alternative dates might well have led to months of electioneering, which "would not be in the public interest".
Scottish National Party leader Alex Salmond claimed Labour had "pressurised" the presiding officer into the decision and said his party now had to abandon plans for a one member one vote ballot to select its candidate.
He said: "It is typical of Labour's chicanery, but they will get their comeuppance from the voters of Cathcart.
"The idea of holding an election 23 days from now, before the ink is even dry on Lord Watson's resignation, shows they are running scared of the prospect of a double defeat by the SNP in Livingston and then Cathcart.
"It is a cheap and blatant attempt to slash and burn Scottish democracy to suit Labour's convenience and a transparent effort to protect the Labour Party from the political consequences of Lord Watson's fire-raising."
Bill Aitken, the Tories' chief whip at Holyrood, added: "Labour are obviously seeking to cut and run.
"And whilst we would prefer more time we look forward to highlighting the way in which the Labour-Lib Dem Executive's policies have failed Glasgow."
Labour said the by-election was an inconvenience for local voters and accused opponents of "hypocrisy" as they had their own "expedient" reasons for preferring separate by-election dates.
Lord Watson, who will be sentenced at Edinburgh Sheriff Court on Wednesday 22 September, held a majority of 5,112 over the SNP in the 2003 Scottish Parliament election.