An SPFL spokesman said: “No new date for this fixture has been set, pending the outcome of the SPFL investigation.”

That decision to delay naming a new date could leave Killie sweating over the outcome of the probe.

No decision has been announced on Kilmarnock’s first two Betfred Cup games, at Falkirk on October 6 and at home to Dunfermline seven days later.

The game was put in doubt on Thursday night after a further three players tested positive 24 hours after the initial cases were confirmed.

An SPFL statement read: “On 29 September, Kilmarnock FC carried out routine swab tests on their first-team squad of players in advance of the fixture this evening.

“This disclosed three players who tested positive for Covid-19. These positive results were subsequently confirmed by NHS Scotland.

“On 1 October, Kilmarnock FC retested all of its remaining first-team players. The results have disclosed a further three first-team squad players who have tested positive for Covid-19.

“A meeting led by NHS Ayrshire & Arran Public Health was held this morning, including representatives from Kilmarnock FC, the Scottish Government, Public Health Scotland and the Scottish FA.

“NHS Ayrshire & Arran Test and Protect Team carried out Covid-19 contact tracing following the six confirmed positive tests at the club.

“As a result, Kilmarnock FC has been advised by the local Problem Assessment Group that their whole first team playing squad require to immediately self-isolate.

“This is in order to minimise the risk of further cases in the player and other staff groups at the club, in the wider local community, and to support Test and Protect.

“Subsequently, Kilmarnock FC wrote to the SPFL indicating that they were not in a position to fulfil this evening’s scheduled fixture against Motherwell and the club requested a postponement of the fixture.

“The SPFL board met this afternoon and has agreed to postpone the fixture whilst the position is investigated.

“An investigation will now be carried out by the SPFL to ascertain the circumstances leading to this postponement.”

Kilmarnock stated that their Rugby Park stadium would now undergo a deep clean.

A club statement added: “The health and wellbeing of our players and staff remains critical and all six individuals are either asymptomatic or are experiencing mild symptoms at this time.”

The SPFL decision will come under scrutiny after St Mirren were ordered to take on Hibernian three weeks ago after all three of their goalkeepers were instructed to self-isolate.

Saints managed to sign Zdenek Zlamal on loan from Hearts, the only club outside the top flight who are testing players, and put goalkeeping coach Jamie Langfield on their bench.

However, the fact that Killie had no players available who had submitted negative tests gave the league a bigger headache.

A number of games involving Aberdeen and Celtic were previously postponed on the advice of the Scottish Government after high-profile breaches of coronavirus protocols from players in each team.

Both clubs were given immediate £8,000 fines by the SPFL while their offending players were all dealt with separately under the Scottish Football Association disciplinary procedures.

The decision to quarantine the entire Killie squad comes 24 hours after a similar instruction at East Stirlingshire was reversed.

The Lowland League club’s entire squad, staff and all their families – 72 people in total – were told to self-isolate after one player tested positive, but those restrictions were lifted after six days.

Meanwhile, the Challenge Cup has been shelved after approval from 34 of the league’s 42 clubs.

A spokesman for the SPFL said: “It is extremely concerning that so many clubs felt that they could not afford to participate in the Challenge Cup this season, given the continuing restrictions on fans coming to matches.

“Fans remain the lifeblood of the game in Scotland. Today’s decision to cancel the Challenge Cup this season is a further demonstration of the urgent need to get fans back into stadia safely, as they are in other countries.

“A continuing ban on fans threatens the very future of many of our clubs which have been mainstays of local communities across the country for generations.”

Last season’s final between Inverness and Raith Rovers will be played at some stage in the upcoming lower-league campaign.