TCS — False confirmation for TCS job

I got placed at TCS in November 2022. After my placement I received offer letter from TCS where joining date was not mentioned.
My previous employer Notice period was 3 months. After receiving the offer letter I was told to wait for resigning my previous job till the background check completes by TCS recruiter.

The background check (BGC) was put on hold by TCS and even after submitting all the documents and proof, I was not receiving any further communication for my BGC completion. I tried to reach them( TCS recruitment team) several times over calls and emails but they were not replying.

My tentative date of joining as asked by them was Mar 2023.As my tentative date of joining was coming near I had dropped them emails several times for providing confirmation on my BGC.

But even afterwards they were not giving me any confirmation and asking me to wait and that they are working on it.

Now when my tentative date of joining is almost near they are saying to on board on TCS else as my offer will stand rejected.

I told them that they asked me to wait serving previous firm notice period till the BGC completes but they are backing off and not agreeing on the same.

As per now they are blaming me completely for not serving the notice period and now that my job offer will be considered rejected as per the offer letter they shared.